Include single_thread_task_runner.h in MessageLoopCurrent since API takes it by value

TBR=fdoray@chromium.org

Bug: 825327
Change-Id: Iead1c89cfc21458e828db8d3b560643dcd088b08
Reviewed-on: https://chromium-review.googlesource.com/1027793Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553562}

base/message_loop/message_loop_current.h

@@ -12,12 +12,12 @@
 #include "base/message_loop/message_pump_for_io.h"
 #include "base/message_loop/message_pump_for_ui.h"
 #include "base/pending_task.h"
+#include "base/single_thread_task_runner.h"
 #include "build/build_config.h"
 
 namespace base {
 
 class MessageLoop;
-class SingleThreadTaskRunner;
 
 // MessageLoopCurrent is a proxy to the public interface of the MessageLoop
 // bound to the thread it's obtained on.