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 115f31a0429ce2209e3f4835794a95f82a7784db..5dc252b12649f16bbb5da21c7e1de0d452f37bd8 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -152,6 +152,12 @@ 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",
@@ -176,6 +182,17 @@ static_library("rtc_base_approved") {
       "logging.h",
       "logging_mac.mm",
     ]
+    if (!is_win && !is_mac && !is_ios) {
+      deps += [ "//base/third_party/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" ]
   }
 }