Allow external TaskQueue implementations on Linux

webrtc/rtc_base/BUILD.gn

Index: webrtc/rtc_base/BUILD.gn
diff --git a/webrtc/rtc_base/BUILD.gn b/webrtc/rtc_base/BUILD.gn
index 60d1f57583cf9f86f0d74ac6c63484d30a5f39c6..7f58e97d4362e193347677eddba0a004f6d5ea1e 100644
--- a/webrtc/rtc_base/BUILD.gn
+++ b/webrtc/rtc_base/BUILD.gn
@@ -301,35 +301,49 @@ rtc_static_library("rtc_task_queue") {
 
   if (build_with_chromium) {
     sources = [
-      "../../webrtc_overrides/webrtc/rtc_base/task_queue.cc",

[nisse-webrtc, 2017/08/22 13:39:44]

Ah, Chrome already overrides task queue, I had forgotten about that. Can we
arrange so that it use the new mechanism too (and not override any header
files)? Otherwise, we get into a mess with multiple ways the task queue
implementation can be replaced.

[perkj_webrtc, 2017/08/22 18:52:38]

yes, if I can get some help.
https://cs.chromium.org/chromium/src/third_party/webrtc_overrides/webrtc/rtc_...

       "../../webrtc_overrides/webrtc/rtc_base/task_queue.h",
     ]
   } else {
     sources = [
       "task_queue.h",
-      "task_queue_posix.h",
     ]
+  }
+  if (rtc_link_task_queue_impl) {
+    deps = [
+      ":rtc_task_queue_impl",
+    ]
+  }
+}
+
+rtc_static_library("rtc_task_queue_impl") {
+  if (build_with_chromium) {
+    sources = [
+      "../../webrtc_overrides/webrtc/rtc_base/task_queue.cc",
+    ]
+  } else {
     if (rtc_build_libevent) {
       deps = [
         "//base/third_party/libevent",
       ]
     }
-
     if (rtc_enable_libevent) {
-      sources += [
+      sources = [
         "task_queue_libevent.cc",
         "task_queue_posix.cc",
+        "task_queue_posix.h",
       ]
       all_dependent_configs = [ ":enable_libevent_config" ]
     } else {
       if (is_mac || is_ios) {
-        sources += [
+        sources = [
           "task_queue_gcd.cc",
           "task_queue_posix.cc",
         ]
       }
       if (is_win) {
-        sources += [ "task_queue_win.cc" ]
+        sources = [
+          "task_queue_win.cc",
+        ]
       }
     }
   }