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* Electron version: 1.7.2
* Operating system: macOS Sierra

### Expected behavior

Webview appear

### Actual behavior

It don't

### How to reproduce

1. Create a browser window with contextIsolation on true and webviewTag on true
2. Inside of it, add a webview
3. Run it, webview should not appear

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atom/renderer/atom_render_frame_observer.cc
IPC::PlatformFileForTransit file_handle,
    const std::string& channel) {
  base::ThreadRestrictions::ScopedAllowIO allow_io;
  auto file = IPC::PlatformFileForTransitToFile(file_handle);
  bool success = TakeHeapSnapshot(blink::MainThreadIsolate(), &file);
  base::ListValue args;
  args.AppendString(channel);
  args.AppendBoolean(success);
  render_frame_->Send(new AtomFrameHostMsg_Message(
      render_frame_->GetRoutingID(), "ipc-message", args));
}
void AtomRenderFrameObserver::EmitIPCEvent(blink::WebLocalFrame* frame,