Reland of New task queueing primitive for async tasks: TaskQueue.

webrtc/build/common.gypi

Index: webrtc/build/common.gypi
diff --git a/webrtc/build/common.gypi b/webrtc/build/common.gypi
index 15aa19155f3d3568fbf31966fcf10b7316c39639..4006d1745aabc7526b5ee111672d9220274765c3 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -19,7 +19,9 @@
 
           # Enable to use the Mozilla internal settings.
           'build_with_mozilla%': 0,
+          'build_for%': '',
         },
+        'build_for%': '<(build_for)',
         'build_with_chromium%': '<(build_with_chromium)',
         'build_with_mozilla%': '<(build_with_mozilla%)',
         'include_opus%': 1,
@@ -41,10 +43,20 @@
             'apk_tests_path%': '<(DEPTH)/webrtc/build/apk_tests.gyp',
             'modules_java_gyp_path%': '<(DEPTH)/webrtc/modules/modules_java.gyp',
           }],
+
+          # Controls whether we use libevent on posix platforms.
+          # TODO(tommi): Remove the 'build_for' condition once libevent is more
+          # widely available in posix configurations.
+          ['OS=="win" or OS=="mac" or OS=="ios" or build_for!=""', {
+            'build_libevent%': 0,
+          }, {
+            'build_libevent%': 1,
+          }],
         ],
       },
       'build_with_chromium%': '<(build_with_chromium)',
       'build_with_mozilla%': '<(build_with_mozilla)',
+      'build_libevent%': '<(build_libevent)',
       'webrtc_root%': '<(webrtc_root)',
       'apk_tests_path%': '<(apk_tests_path)',
       'modules_java_gyp_path%': '<(modules_java_gyp_path)',
@@ -56,6 +68,7 @@
     },
     'build_with_chromium%': '<(build_with_chromium)',
     'build_with_mozilla%': '<(build_with_mozilla)',
+    'build_libevent%': '<(build_libevent)',
     'webrtc_root%': '<(webrtc_root)',
     'apk_tests_path%': '<(apk_tests_path)',
     'test_runner_path': '<(DEPTH)/webrtc/build/android/test_runner.py',
@@ -319,6 +332,11 @@
           }],
         ],
       }],
+      ['build_libevent==1', {
+        'defines': [
+          'WEBRTC_BUILD_LIBEVENT',
+        ],
+      }],
       ['target_arch=="arm64"', {
         'defines': [
           'WEBRTC_ARCH_ARM64',