Introduce XGetImage() for GrabWindowSnapshot() in ChromeOS.

BUG=119492
TEST=manually done

Review URL: https://chromiumcodereview.appspot.com/10386124

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@137556 0039d316-1c4b-4281-b951-d872f2087c98
void RootWindowHostLinux::Show() {
  XSizeHints size_hints;
  size_hints.flags = PPosition;
  size_hints.x = bounds_.x();
  size_hints.y = bounds_.y();
  XSetWMNormalHints(xdisplay_, xwindow_, &size_hints);

  XMapWindow(xdisplay_, xwindow_);
}

void RootWindowHostLinux::Show() {
  XSizeHints size_hints;
  size_hints.flags = PPosition;
  size_hints.x = bounds_.x();
  size_hints.y = bounds_.y();
  XSetWMNormalHints(xdisplay_, xwindow_, &size_hints);

  XMapWindow(xdisplay_, xwindow_);
}
