Remove an unused forward decl in //base/timer/timer.h

Tbr: jennyz@chromium.org
Change-Id: Iac44f0b08ffef83322e9811fe4bc65cab11ce1c1
Reviewed-on: https://chromium-review.googlesource.com/940706
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539787}

base/timer/timer.h

@@ -78,10 +78,6 @@ namespace base {
 class BaseTimerTaskInternal;
 class TickClock;
 
-// TODO(gab): Removing this fwd-decl causes IWYU failures in other headers,
-// remove it in a follow- up CL.
-class SingleThreadTaskRunner;
-
 //-----------------------------------------------------------------------------
 // This class wraps TaskRunner::PostDelayedTask to manage delayed and repeating
 // tasks. See meta comment above for thread-safety requirements.

chromeos/audio/cras_audio_handler.h

@@ -26,6 +26,10 @@
 #include "chromeos/dbus/volume_state.h"
 #include "media/base/video_facing.h"
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace chromeos {
 
 class AudioDevicesPrefHandler;