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 cd7e5b51e598d40cdfce7e8be807309456bcd74c..cc8b76b78bf7bb3bd3934df90328df7797d5822b 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -86,6 +86,7 @@ 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" ]
@@ -147,6 +148,12 @@ 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",
@@ -172,6 +179,23 @@ static_library("rtc_base_approved") {
       "logging_mac.mm",
     ]
   }
+
+  if (!is_win && !is_mac && !is_ios && !is_nacl) {

[kjellander_webrtc, 2016/05/16 12:25:50]

For GN, please introduce the same variables as for the GYP build, in
webrtc/build/webrtc.gni.

+    deps += [ "//base/third_party/libevent" ]
+    defines += [ "WEBRTC_BUILD_LIBEVENT" ]
+  }
+
+  if (is_mac || is_ios || is_win || is_nacl) {
+    sources -= [ "task_queue_libevent.cc" ]

[kjellander_webrtc, 2016/05/16 12:25:50]

Please only add to sources, i.e. avoid using subtraction from sources listings
(applies to GYP as well).
I've spend a fair amount of time cleaning up webrtc/base from this (see  
https://codereview.webrtc.org/1857163003 for a painful CL) and I don't want to
see it regress again. Maintenance become very cumbersome and error prone.

+  }
+
+  if (is_linux || is_android || is_win || is_nacl) {
+    sources -= [ "task_queue_gcd.cc" ]
+  }
+
+  if (is_nacl) {
+    sources -= [ "task_queue_posix.cc" ]
+  }
 }
 
 static_library("rtc_base") {