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::StopMediaSession() {
  MediaSession::Get(this)->Stop();
}

void WebContentsImpl::StopMediaSession() {
  MediaSession::Get(this)->Stop();
}
