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.h
void DidStartLoading() override;
  void DidStopLoading() override;
  void DidGetResourceResponseStart(
      const content::ResourceRequestDetails& details) override;
  void DidGetRedirectForResourceRequest(
      content::RenderFrameHost* render_frame_host,
      const content::ResourceRedirectDetails& details) override;
  void DidFinishNavigation(
      content::NavigationHandle* navigation_handle) override;
  bool OnMessageReceived(const IPC::Message& message) override;
  void WebContentsDestroyed() override;
  void NavigationEntryCommitted(
      const content::LoadCommittedDetails& load_details) override;
  void TitleWasSet(content::NavigationEntry* entry, bool explicit_set) override;
  void DidUpdateFaviconURL(
      const std::vector<content::FaviconURL>& urls) override;
  void PluginCrashed(const base::FilePath& plugin_path,
                     base::ProcessId plugin_pid) override;
  void MediaStartedPlaying(const MediaPlayerId& id) override;
  void MediaStoppedPlaying(const MediaPlayerId& id) override;
  void DidChangeThemeColor(SkColor theme_color) override;
  
  void DevToolsReloadPage() override;
  
  void DevToolsFocused() override;
  void DevToolsOpened() override;
  void DevToolsClosed() override;
 private:
  AtomBrowserContext* GetBrowserContext() const;
  uint32_t GetNextRequestId() {