[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}
WebContentsImpl* PageHandler::GetWebContents() {
  return host_ && !host_->frame_tree_node()->parent()
             ? static_cast<WebContentsImpl*>(
                   WebContents::FromRenderFrameHost(host_))
             : nullptr;
}

WebContentsImpl* PageHandler::GetWebContents() {
  return host_ && !host_->frame_tree_node()->parent()
             ? static_cast<WebContentsImpl*>(
                   WebContents::FromRenderFrameHost(host_))
             : nullptr;
}
