[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}
void RenderFrameDevToolsAgentHost::DestroyOnRenderFrameGone() {
  scoped_refptr<RenderFrameDevToolsAgentHost> protect(this);
  if (IsAttached())
    RevokePolicy();
  ForceDetachAllSessions();
  frame_host_ = nullptr;
  UpdateRendererChannel(IsAttached());
  SetFrameTreeNode(nullptr);
  Release();
}

void RenderFrameDevToolsAgentHost::DestroyOnRenderFrameGone() {
  scoped_refptr<RenderFrameDevToolsAgentHost> protect(this);
  if (IsAttached())
    RevokePolicy();
  ForceDetachAllSessions();
  frame_host_ = nullptr;
  UpdateRendererChannel(IsAttached());
  SetFrameTreeNode(nullptr);
  Release();
}
