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
return type_ == BROWSER_WINDOW;
}
void WebContents::HandleKeyboardEvent(
    content::WebContents* source,
    const content::NativeWebKeyboardEvent& event) {
  if (type_ == WEB_VIEW && embedder_) {
    
    embedder_->HandleKeyboardEvent(source, event);
  } else {
    
    CommonWebContentsDelegate::HandleKeyboardEvent(source, event);
  }
}
void WebContents::EnterFullscreenModeForTab(content::WebContents* source,
                                            const GURL& origin) {
  auto permission_helper =
      WebContentsPermissionHelper::FromWebContents(source);
  auto callback = base::Bind(&WebContents::OnEnterFullscreenModeForTab,
                             base::Unretained(this), source, origin);
  permission_helper->RequestFullscreenPermission(callback);
}
void WebContents::OnEnterFullscreenModeForTab(content::WebContents* source,
                                              const GURL& origin,
                                              bool allowed) {
  if (!allowed)
    return;
  CommonWebContentsDelegate::EnterFullscreenModeForTab(source, origin);
  Emit("enter-html-full-screen");
}
void WebContents::ExitFullscreenModeForTab(content::WebContents* source) {