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
const base::FilePath& path) {
  if (path.empty()) {
    args->ThrowError("path cannot be empty");
    return;
  }
  DevToolsRemoveFileSystem(path);
}
void WebContents::Undo() {
  web_contents()->Undo();
}
void WebContents::Redo() {
  web_contents()->Redo();
}
void WebContents::Cut() {
  web_contents()->Cut();
}
void WebContents::Copy() {
  web_contents()->Copy();
}
void WebContents::Paste() {
  web_contents()->Paste();
}
void WebContents::PasteAndMatchStyle() {