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}
bool RenderWidgetHostImpl::CanCopyFromBackingStore() {
  if (view_)
    return view_->IsSurfaceAvailableForCopy();
  return false;
}

bool RenderWidgetHostImpl::CanCopyFromBackingStore() {
  if (view_)
    return view_->IsSurfaceAvailableForCopy();
  return false;
}
