I tired with the offscreen mode and here are the problems I found:
- [x] <del>The `data` of `paint` event is empty when loading simple local pages.</del>
- [x] The size of `data` is wrong, current implement uses the size of pointer (`sizeof(bitmap_pixels)`).
- [x] Crashes when opening devtools for offscreen window.
- [x] We should probably show a dummy view indicating it is offscreen instead of provide NULL.
- [x] The offscreen window is affecting the rendering of other windows. Try running `./script/test.py -g offscreen` and then refreshing the window.

/cc @gerhardberger @brenca

atom/browser/api/atom_api_web_contents.cc
Emit("devtools-reload-page");
}
void WebContents::DevToolsFocused() {
  Emit("devtools-focused");
}
void WebContents::DevToolsOpened() {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto handle = WebContents::CreateFrom(
      isolate(), managed_web_contents()->GetDevToolsWebContents());
  devtools_web_contents_.Reset(isolate(), handle.ToV8());
  
  base::FundamentalValue tab_id(ID());
  managed_web_contents()->CallClientFunction(
      "DevToolsAPI.setInspectedTabId", &tab_id, nullptr, nullptr);
  
  if (owner_window())
    handle->SetOwnerWindow(managed_web_contents()->GetDevToolsWebContents(),
                           owner_window());
  Emit("devtools-opened");
}
void WebContents::DevToolsClosed() {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  devtools_web_contents_.Reset();
  Emit("devtools-closed");
}
bool WebContents::OnMessageReceived(const IPC::Message& message) {