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

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index 63c2da533e5ab3cf3e40292e411105f6cceba9c4..1cb13ef47c1dc493b9563508071a9e5c5821f3d4 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -92,6 +92,7 @@ if (rtc_build_ssl == 0) {
 
 # The subset of rtc_base approved for use outside of libjingle.
 static_library("rtc_base_approved") {
+  defines = []
   deps = []
   configs += [ "..:common_config" ]
   public_configs = [ "..:common_inherited_config" ]
@@ -153,6 +154,12 @@ static_library("rtc_base_approved") {
     "swap_queue.h",
     "systeminfo.cc",
     "systeminfo.h",
+    "task_queue.h",
+    "task_queue_gcd.cc",
+    "task_queue_libevent.cc",
+    "task_queue_posix.cc",
+    "task_queue_posix.h",
+    "task_queue_win.cc",
     "template_util.h",
     "thread_annotations.h",
     "thread_checker.h",
@@ -178,6 +185,19 @@ static_library("rtc_base_approved") {
       "logging_mac.mm",
     ]
   }
+
+  if (!is_win && !is_mac && !is_ios) {
+    deps += [ "//base/third_party/libevent" ]
+    defines += [ "WEBRTC_BUILD_LIBEVENT" ]
+  }
+
+  if (is_mac || is_ios || is_win) {
+    sources -= [ "task_queue_libevent.cc" ]
+  }
+
+  if (is_linux || is_android || is_win) {
+    sources -= [ "task_queue_gcd.cc" ]
+  }
 }
 
 static_library("rtc_base") {