Fix GYP and GN references that are invalid in Chromium builds.

There were a couple of GN and GYP references that were incorrect in Chromium builds:
- GN references between WebRTC targets must be using relative paths, not absolute.
- GYP references between WebRTC targets must be using the <(webrtc_root)v variable
  in order to be expanded to the correct path in a Chromium build.

NOTRY=True
TBR=hjon@webrtc.org, hbos@webrtc.org

Review URL: https://codereview.webrtc.org/1681493002

Cr-Commit-Position: refs/heads/master@{#11521}

talk/build/merge_ios_libs.gyp

@@ -37,7 +37,7 @@
             '<(webrtc_root)/system_wrappers/system_wrappers.gyp:field_trial_default',
             '../libjingle.gyp:libjingle_peerconnection_objc',
           ],
-          'sources': ['<(DEPTH)/webrtc/build/no_op.cc',],
+          'sources': ['<(webrtc_root)/build/no_op.cc',],
         },
       ],
     }]

webrtc/api/BUILD.gn

@@ -20,8 +20,8 @@ config("ios_config") {
 if (is_ios) {
   source_set("rtc_api_objc") {
     deps = [
-      "//webrtc/base:rtc_base_objc",
-      #"//talk/libjingle:libjingle_peerconnection",
+      "../base:rtc_base_objc",
+      #"../../talk/libjingle:libjingle_peerconnection",
     ]
     cflags = [
       "-fobjc-arc",

webrtc/modules/video_coding/BUILD.gn

@@ -150,9 +150,9 @@ source_set("webrtc_h264") {
       "codecs/h264/h264_encoder_impl.h",
     ]
     deps += [
+      "../../common_video",
       "//third_party/ffmpeg:ffmpeg",
       "//third_party/openh264:encoder",
-      "//webrtc/common_video",
     ]
   }
 }

webrtc/modules/video_coding/codecs/h264/h264.gypi

@@ -40,7 +40,7 @@
           'dependencies': [
             '<(DEPTH)/third_party/ffmpeg/ffmpeg.gyp:ffmpeg',
             '<(DEPTH)/third_party/openh264/openh264.gyp:openh264_encoder',
-            '<(DEPTH)/webrtc/common_video/common_video.gyp:common_video',
+            '<(webrtc_root)/common_video/common_video.gyp:common_video',
           ],
           'sources': [
             'h264_decoder_impl.cc',

webrtc/test/test.gyp

@@ -83,8 +83,8 @@
         'rtp_file_writer.h',
       ],
       'dependencies': [
-        '<(DEPTH)/webrtc/common.gyp:webrtc_common',
         '<(DEPTH)/testing/gtest.gyp:gtest',
+        '<(webrtc_root)/common.gyp:webrtc_common',
         '<(webrtc_root)/modules/modules.gyp:rtp_rtcp',
       ],
     },