* Electron version: 1.7.6
* Operating system: macOS 10.12, Windows 10

### Expected behavior

Right clicking should open context menu in devtools

### Actual behavior

Right clicking does not open the context menu in devtools

### How to reproduce

- `open Electron.app --args --enable-mixed-sandbox`
- open devtools
- right click in the source tab for example
atom/renderer/atom_sandboxed_renderer_client.cc
return base::CommandLine::ForCurrentProcess()->argv();
}
void InitializeBindings(v8::Local<v8::Object> binding,
                        v8::Local<v8::Context> context) {
  auto isolate = context->GetIsolate();
  mate::Dictionary b(isolate, binding);
  b.SetMethod("get", GetBinding);
  b.SetMethod("crash", AtomBindings::Crash);
  b.SetMethod("hang", AtomBindings::Hang);
  b.SetMethod("getArgv", GetArgv);
  b.SetMethod("getProcessMemoryInfo", &AtomBindings::GetProcessMemoryInfo);
  b.SetMethod("getSystemMemoryInfo", &AtomBindings::GetSystemMemoryInfo);
}
class AtomSandboxedRenderFrameObserver : public AtomRenderFrameObserver {
 public:
  AtomSandboxedRenderFrameObserver(content::RenderFrame* render_frame,
                                   AtomSandboxedRendererClient* renderer_client)
      : AtomRenderFrameObserver(render_frame, renderer_client),
        v8_converter_(new atom::V8ValueConverter),
        renderer_client_(renderer_client) {
    v8_converter_->SetDisableNode(true);
  }
 protected:
  void EmitIPCEvent(blink::WebLocalFrame* frame,