Update DCHECK to CHECK for webview begin frame issue

I suspect that we are encountering back to back begin frame requests
that cause use to write out two begin frame responses which cause
us to crash. Ensure that the begin_frame_response_valid_ is false
when we generate a BeginFrame.

BUG=885124

Change-Id: I21332bccaea142f8daf18a15805142f03f59c339
Reviewed-on: https://chromium-review.googlesource.com/1231733Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592390}

content/browser/android/synchronous_compositor_sync_call_bridge.cc

@@ -85,7 +85,7 @@ bool SynchronousCompositorSyncCallBridge::WaitAfterVSyncOnUIThread(
   base::AutoLock lock(lock_);
   if (remote_state_ != RemoteState::READY)
     return false;
-  DCHECK(!begin_frame_response_valid_);
+  CHECK(!begin_frame_response_valid_);
   if (window_android_in_vsync_) {
     DCHECK_EQ(window_android_in_vsync_, window_android);
     return true;