Early return in {Suspend,Resume}Renderer() if not backgrounded

After crrev.com/2564833002, RenderThreadImpl calls SuspendRenderer() and
ResumeRenderer() unconditionally. This could cause assertion failures
when the renderer isn't backgrounded.  Rather than reverting
crrev.com/2564833002, do early return when not backgrounded because
having multiple flags to check whether a renderer is suspended doesn't
sound a good idea.

BUG=674784

Review-Url: https://codereview.chromium.org/2583083002
Cr-Commit-Position: refs/heads/master@{#439436}

third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc

@@ -549,9 +549,10 @@ void RendererSchedulerImpl::OnAudioStateChanged() {
 
 void RendererSchedulerImpl::SuspendRenderer() {
   helper_.CheckOnValidThread();
-  DCHECK(MainThreadOnly().renderer_backgrounded);
   if (helper_.IsShutdown())
     return;
+  if (!MainThreadOnly().renderer_backgrounded)
+    return;
   suspend_timers_when_backgrounded_closure_.Cancel();
 
   UMA_HISTOGRAM_COUNTS("PurgeAndSuspend.PendingTaskCount",
@@ -565,9 +566,10 @@ void RendererSchedulerImpl::SuspendRenderer() {
 
 void RendererSchedulerImpl::ResumeRenderer() {
   helper_.CheckOnValidThread();
-  DCHECK(MainThreadOnly().renderer_backgrounded);
   if (helper_.IsShutdown())
     return;
+  if (!MainThreadOnly().renderer_backgrounded)
+    return;
   suspend_timers_when_backgrounded_closure_.Cancel();
   MainThreadOnly().renderer_suspended = false;
   ResumeTimerQueueWhenForegroundedOrResumed();