Use DISPATCH_QUEUE_SERIAL constant in dispatch_source_mach.cc.

Change-Id: I220dd3b48482e26d94d4f2a8a8fcd08097038a9d
Reviewed-on: https://chromium-review.googlesource.com/1183701Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584801}

base/mac/dispatch_source_mach.cc

@@ -9,9 +9,7 @@ namespace base {
 DispatchSourceMach::DispatchSourceMach(const char* name,
                                        mach_port_t port,
                                        void (^event_handler)())
-    // TODO(rsesek): Specify DISPATCH_QUEUE_SERIAL, in the 10.7 SDK. NULL
-    // means the same thing but is not symbolically clear.
-    : DispatchSourceMach(dispatch_queue_create(name, NULL),
+    : DispatchSourceMach(dispatch_queue_create(name, DISPATCH_QUEUE_SERIAL),
                          port,
                          event_handler) {
   // Since the queue was created above in the delegated constructor, and it was