Split TaskQueue out into a separate build target.

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index 1e868b504e528e911118efd52f261bf14129fff9..9a97232fa536f3a83ecec31b435b95cc46e0a62f 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -147,8 +147,6 @@ static_library("rtc_base_approved") {
     "swap_queue.h",
     "systeminfo.cc",
     "systeminfo.h",
-    "task_queue.h",
-    "task_queue_posix.h",
     "template_util.h",
     "thread_annotations.h",
     "thread_checker.h",
@@ -174,16 +172,22 @@ static_library("rtc_base_approved") {
       "logging_mac.mm",
     ]
   }
+}
 
-  if (rtc_build_libevent) {
-    deps += [ "//base/third_party/libevent" ]
-  }
-  if (rtc_enable_libevent) {
-    sources += [
-      "task_queue_libevent.cc",
-      "task_queue_posix.cc",
-    ]
-  } else {
+static_library("rtc_task_queue") {
+  defines = []
+  deps = []
+  public_deps = [
+    ":rtc_base_approved",
+  ]
+
+  sources = [
+    "task_queue.h",
+    "task_queue_posix.h",
+  ]
+
+  # Enable libevent task queues on platforms that support it.
+  if (is_win || is_mac || is_ios) {
     # If not libevent, fall back to the other task queues.
     if (is_mac || is_ios) {
       sources += [
@@ -194,6 +198,13 @@ static_library("rtc_base_approved") {
     if (is_win) {
       sources += [ "task_queue_win.cc" ]
     }
+  } else {
+    defines += [ "WEBRTC_BUILD_LIBEVENT" ]

[phoglund, 2016/05/23 08:23:32]

Please move this back where it was; we need GN and GYP to look the same. Also
this is brittle since the define needs to be on all targets that include
task_queue.h (or we hit the #error in there).

[tommi, 2016/05/23 15:37:19]

Done.

+    deps += [ "//base/third_party/libevent" ]

[phoglund, 2016/05/23 08:23:32]

This no longer respects rtc_build_libevent which means you'll break downstream.

+    sources += [
+      "task_queue_libevent.cc",
+      "task_queue_posix.cc",
+    ]
   }
 }