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::ClearDisplayedGraphics() {
  NotifyNewContentRenderingTimeoutForTesting();
  if (view_)
    view_->ClearCompositorFrame();
}

void RenderWidgetHostImpl::ClearDisplayedGraphics() {
  NotifyNewContentRenderingTimeoutForTesting();
  if (view_)
    view_->ClearCompositorFrame();
}
