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::MediaStartedPlaying(
    const WebContentsObserver::MediaPlayerId& id) {
  FOR_EACH_OBSERVER(WebContentsObserver, observers_, MediaStartedPlaying(id));
}

void WebContentsImpl::MediaStartedPlaying(
    const WebContentsObserver::MediaPlayerId& id) {
  FOR_EACH_OBSERVER(WebContentsObserver, observers_, MediaStartedPlaying(id));
}
