Don't call WebContents::DownloadImage() callback if the WebContents were deleted

BUG=583718

Review URL: https://codereview.chromium.org/1685343004

Cr-Commit-Position: refs/heads/master@{#375700}
void WebContentsImpl::WasUnOccluded() {
  for (RenderWidgetHostView* view : GetRenderWidgetHostViewsInTree()) {
    if (view)
      view->WasUnOccluded();
  }
}

void WebContentsImpl::WasUnOccluded() {
  for (RenderWidgetHostView* view : GetRenderWidgetHostViewsInTree()) {
    if (view)
      view->WasUnOccluded();
  }
}
