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
}
void WebContents::RenderViewDeleted(content::RenderViewHost* render_view_host) {
  Emit("render-view-deleted", render_view_host->GetProcess()->GetID());
}
void WebContents::RenderProcessGone(base::TerminationStatus status) {
  Emit("crashed");
}
void WebContents::PluginCrashed(const base::FilePath& plugin_path,
                                base::ProcessId plugin_pid) {
  content::WebPluginInfo info;
  auto plugin_service = content::PluginService::GetInstance();
  plugin_service->GetPluginInfoByPath(plugin_path, &info);
  Emit("plugin-crashed", info.name, info.version);
}
void WebContents::MediaStartedPlaying(const MediaPlayerId& id) {
  Emit("media-started-playing");
}
void WebContents::MediaStoppedPlaying(const MediaPlayerId& id) {
  Emit("media-paused");
}
void WebContents::DidChangeThemeColor(SkColor theme_color) {
  std::string hex_theme_color = base::StringPrintf("#%02X%02X%02X",
    SkColorGetR(theme_color),
    SkColorGetG(theme_color),
    SkColorGetB(theme_color));
  Emit("did-change-theme-color", hex_theme_color);
}
void WebContents::DocumentLoadedInFrame(