Make sad frame background gray

Less jarring on a page with white background.

Bug: 884819
Change-Id: I40ef521090d6146ba0abe2f78de0656d824988ef
Reviewed-on: https://chromium-review.googlesource.com/1247030Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594529}

content/renderer/child_frame_compositing_helper.cc

@@ -39,7 +39,7 @@ void ChildFrameCompositingHelper::ChildFrameGone(
   scoped_refptr<cc::SolidColorLayer> crashed_layer =
       cc::SolidColorLayer::Create();
   crashed_layer->SetMasksToBounds(true);
-  crashed_layer->SetBackgroundColor(SK_ColorBLACK);
+  crashed_layer->SetBackgroundColor(SK_ColorGRAY);
 
   if (child_frame_compositor_->GetLayer()) {
     SkBitmap* sad_bitmap = child_frame_compositor_->GetSadPageBitmap();