Revert of Remove the rtc_relative_path GYP variable and similar defines (patchset #1 id:1 of https://codereview.webrtc.org/1925733002/ )

Reason for revert:
Breaks downstream gtest usage.

Original issue's description:
> Remove the rtc_relative_path GYP variable and similar defines
>
> This is a reland of https://codereview.webrtc.org/1903553003/ but with
> the SRTP changes removed, since they're needed downstream.
>
> The defines that can be used to alter the include paths for Expat and gtest
> are no longer needed in WebRTC or Chromium. Remove them to simplify GYP.
>
> Removed defines:
> EXPAT_RELATIVE_PATH
> GTEST_RELATIVE_PATH
>
> They're all set in the Chromium build so this shouldn't affect Chromium:
> https://code.google.com/p/chromium/codesearch#chromium/src/third_party/libjingle/libjingle.gyp
>
> BUG=webrtc:4256
> NOTRY=True
> NOPRESUBMIT=True
> TBR=perkj@webrtc.org
>
> Committed: https://crrev.com/081254f2c62037d016f9fc961764c6f01cb095da
> Cr-Commit-Position: refs/heads/master@{#12536}

TBR=perkj@webrtc.org,kjellander@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:4256

Review-Url: https://codereview.webrtc.org/1945803003
Cr-Commit-Position: refs/heads/master@{#12622}

webrtc/base/base_tests.gyp

@@ -23,11 +23,19 @@
         'testechoserver.h',
         'testutils.h',
       ],
+      'defines': [
+        'GTEST_RELATIVE_PATH',
+      ],
       'dependencies': [
         'base.gyp:rtc_base',
         '<(DEPTH)/testing/gtest.gyp:gtest',
         '<(webrtc_root)/test/test.gyp:field_trial',
       ],
+      'direct_dependent_settings': {
+        'defines': [
+          'GTEST_RELATIVE_PATH',
+        ],
+      },
       'export_dependent_settings': [
         '<(DEPTH)/testing/gtest.gyp:gtest',
       ],

webrtc/base/gunit.h

@@ -13,7 +13,11 @@
 
 #include "webrtc/base/logging.h"
 #include "webrtc/base/thread.h"
+#if defined(GTEST_RELATIVE_PATH)
 #include "testing/gtest/include/gtest/gtest.h"
+#else
+#include "testing/base/public/gunit.h"
+#endif
 
 // Wait until "ex" is true, or "timeout" expires.
 #define WAIT(ex, timeout)                                                     \

webrtc/base/gunit_prod.h

@@ -15,7 +15,10 @@
 // Android doesn't use gtest at all, so anything that relies on gtest should
 // check this define first.
 #define NO_GTEST
-#include "testing/gtest/include/gtest/gtest_prod.h"
+#elif defined (GTEST_RELATIVE_PATH)
+#include "gtest/gtest_prod.h"
+#else
+#include "testing/base/gunit_prod.h"
 #endif
 
 #endif  // WEBRTC_BASE_GUNIT_PROD_H_

webrtc/build/common.gypi

@@ -64,6 +64,7 @@
     'webrtc_vp9_dir%': '<(webrtc_vp9_dir)',
     'include_ilbc%': '<(include_ilbc)',
     'include_opus%': '<(include_opus)',
+    'rtc_relative_path%': 1,
     'external_libraries%': '0',
     'json_root%': '<(DEPTH)/third_party/jsoncpp/source/include/',
     # openssl needs to be defined or gyp will complain. Is is only used when
@@ -237,6 +238,9 @@
            '<!@(pkg-config --cflags dbus-glib-1)',
          ],
       }],
+      ['rtc_relative_path==1', {
+        'defines': ['EXPAT_RELATIVE_PATH',],
+      }],
       ['os_posix==1', {
         'configurations': {
           'Debug_Base': {

webrtc/libjingle/xmllite/xmlbuilder.h

@@ -17,7 +17,11 @@
 #include "webrtc/libjingle/xmllite/xmlparser.h"
 #include "webrtc/base/scoped_ptr.h"
 
+#ifdef EXPAT_RELATIVE_PATH
 #include "expat.h"
+#else
+#include "third_party/expat/v2_0_1/Source/lib/expat.h"
+#endif  // EXPAT_RELATIVE_PATH
 
 namespace buzz {
 

webrtc/libjingle/xmllite/xmlparser.h

@@ -14,7 +14,11 @@
 #include <string>
 
 #include "webrtc/libjingle/xmllite/xmlnsstack.h"
+#ifdef EXPAT_RELATIVE_PATH
 #include "expat.h"
+#else
+#include "third_party/expat/v2_0_1/Source/lib/expat.h"
+#endif  // EXPAT_RELATIVE_PATH
 
 struct XML_ParserStruct;
 typedef struct XML_ParserStruct* XML_Parser;

webrtc/libjingle/xmpp/xmppstanzaparser.cc

@@ -13,7 +13,11 @@
 #include "webrtc/libjingle/xmllite/xmlelement.h"
 #include "webrtc/libjingle/xmpp/constants.h"
 #include "webrtc/base/common.h"
+#ifdef EXPAT_RELATIVE_PATH
 #include "expat.h"
+#else
+#include "third_party/expat/v2_0_1/Source/lib/expat.h"
+#endif
 
 namespace buzz {
 

webrtc/webrtc_tests.gypi

@@ -269,6 +269,9 @@
         'base/socketaddress_unittest.cc',
         'base/virtualsocket_unittest.cc',
       ],
+      'defines': [
+        'GTEST_RELATIVE_PATH',
+      ],
       'dependencies': [
         '<(DEPTH)/testing/gtest.gyp:gtest',
         'base/base.gyp:rtc_base',