Check that RWHI isn't deleted manually while owned by a scoped_ptr in RVHI

BUG=590284

Review URL: https://codereview.chromium.org/1747183002

Cr-Commit-Position: refs/heads/master@{#378844}
void RenderWidgetHostImpl::SetBackgroundOpaque(bool opaque) {
  Send(new ViewMsg_SetBackgroundOpaque(GetRoutingID(), opaque));
}

void RenderWidgetHostImpl::SetBackgroundOpaque(bool opaque) {
  Send(new ViewMsg_SetBackgroundOpaque(GetRoutingID(), opaque));
}
