[DevTools] Do not allow Page.setDownloadBehavior for extensions

Bug: 866426
Change-Id: I71b672978e1a8ec779ede49da16b21198567d3a4
Reviewed-on: https://chromium-review.googlesource.com/c/1270007
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598004}
Response PageHandler::Enable() {
  enabled_ = true;
  if (GetWebContents() && GetWebContents()->ShowingInterstitialPage())
    frontend_->InterstitialShown();
  return Response::FallThrough();
}

Response PageHandler::Enable() {
  enabled_ = true;
  if (GetWebContents() && GetWebContents()->ShowingInterstitialPage())
    frontend_->InterstitialShown();
  return Response::FallThrough();
}
