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 f8446d9f91bce9e16aa52cd9dfc5df9ae293007e..77feb4bcad5807d136849ebe2aeb8995803a7e2e 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -41,10 +41,18 @@
             '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.
+          ['OS=="win" or OS=="mac" or OS=="ios"', {
+            '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 +64,7 @@
     },
     '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',
@@ -317,6 +326,11 @@
           }],
         ],
       }],
+      ['build_libevent==1', {
+        'defines': [
+          'WEBRTC_BUILD_LIBEVENT',
+        ],
+      }],
       ['target_arch=="arm64"', {
         'defines': [
           'WEBRTC_ARCH_ARM64',