Put the resources_dir flag into its own target.

I had to change approach. Unfortunately we can't expect
that test_main_lib users link with fileutils, which causes it to
not link when the override symbol is missing.

New approach: resources_dir_flag is now a separate target, it
will be depended upon by the downstream override, which just
reads the flag and returns it as the resource dir. This gets
rid of the mutable state downstream as well.

So:
1) Land this
2) Make downstream read the flag instead of keeping its own state
3) Remove OverrideResourcesDir upstream and clean up the hacks
4) Remove the now orphaned OverrideResourcesDir downstream

Bug: webrtc:9792
Change-Id: Ic2ef3910bb5d39d9fb71e06fbbbb6aec4de52e78
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157041
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29487}

test/BUILD.gn

@@ -255,6 +255,7 @@ if (rtc_include_tests) {
       ":field_trial",
       ":fileutils_override_api",
       ":perf_test",
+      ":resources_dir_flag",
       ":test_support",
       "../rtc_base",
       "../rtc_base:checks",
@@ -500,6 +501,18 @@ rtc_source_set("fileutils") {
   }
 }
 
+rtc_source_set("resources_dir_flag") {
+  testonly = true
+  visibility = [ "*" ]
+  sources = [
+    "testsupport/resources_dir_flag.cc",
+    "testsupport/resources_dir_flag.h",
+  ]
+  deps = [
+    "//third_party/abseil-cpp/absl/flags:flag",
+  ]
+}
+
 # We separate header into own target to make it possible for downstream
 # projects to override implementation.
 rtc_source_set("fileutils_override_api") {

test/test_main_lib.cc

@@ -30,6 +30,7 @@
 #include "test/testsupport/file_utils.h"
 #include "test/testsupport/file_utils_override.h"
 #include "test/testsupport/perf_test.h"
+#include "test/testsupport/resources_dir_flag.h"
 
 #if defined(WEBRTC_WIN)
 #include "rtc_base/win32_socket_init.h"
@@ -81,14 +82,6 @@ ABSL_FLAG(std::vector<std::string>,
 
 #endif
 
-ABSL_FLAG(std::string,
-          resources_dir,
-          "",
-          "Where to look for the runtime dependencies. If not specified, we "
-          "will use a reasonable default depending on where we are running. "
-          "This flag is useful if we copy over test resources to a phone and "
-          "need to tell the tests where their resources are.");
-
 ABSL_FLAG(bool, logs, true, "print logs to stderr");
 ABSL_FLAG(bool, verbose, false, "verbose logs to stderr");
 

test/testsupport/resources_dir_flag.cc

@@ -0,0 +1,19 @@
+/*
+ *  Copyright (c) 2019 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "test/testsupport/resources_dir_flag.h"
+
+ABSL_FLAG(std::string,
+          resources_dir,
+          "",
+          "Where to look for the runtime dependencies. If not specified, we "
+          "will use a reasonable default depending on where we are running. "
+          "This flag is useful if we copy over test resources to a phone and "
+          "need to tell the tests where their resources are.");

test/testsupport/resources_dir_flag.h

@@ -0,0 +1,20 @@
+/*
+ *  Copyright (c) 2019 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include <string>
+
+#ifndef TEST_TESTSUPPORT_RESOURCES_DIR_FLAG_H__
+#define TEST_TESTSUPPORT_RESOURCES_DIR_FLAG_H__
+
+#include "absl/flags/flag.h"
+
+ABSL_DECLARE_FLAG(std::string, resources_dir);
+
+#endif  // TEST_TESTSUPPORT_RESOURCES_DIR_FLAG_H__