Surface synchronization: Fix RWHVChildFrame tear down crash

During tear down, surfaces will be garbage collected which may force
other surfaces to activate. However, during tear down, a
RenderWidgetHostViewChildFrame may no longer have a CrossProcessFrameConnector
and so it cannot send the new SurfaceInfo anywhere. If RWHVChildFrame does
not have a CPFC then we should just simply drop the SurfaceInfo.

Bug: 797801
Change-Id: I4b22af61ebc786dcc285311512cbc81bb570ba74
Reviewed-on: https://chromium-review.googlesource.com/849132Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526730}

content/browser/renderer_host/render_widget_host_view_child_frame.cc

@@ -605,7 +605,8 @@ void RenderWidgetHostViewChildFrame::SendSurfaceInfoToEmbedder() {
 void RenderWidgetHostViewChildFrame::SendSurfaceInfoToEmbedderImpl(
     const viz::SurfaceInfo& surface_info,
     const viz::SurfaceSequence& sequence) {
-  frame_connector_->SetChildFrameSurface(surface_info, sequence);
+  if (frame_connector_)
+    frame_connector_->SetChildFrameSurface(surface_info, sequence);
 }
 
 void RenderWidgetHostViewChildFrame::SubmitCompositorFrame(