Introduce XGetImage() for GrabWindowSnapshot() in ChromeOS.

ui/aura/root_window.cc

Index: ui/aura/root_window.cc
diff --git a/ui/aura/root_window.cc b/ui/aura/root_window.cc
index ad3d3efb5ecc05b946b99c56785c7cf7f3ff5d96..e6797dcd024fa26ceb2e6e6fe40a9336e38d2018 100644
--- a/ui/aura/root_window.cc
+++ b/ui/aura/root_window.cc
@@ -532,6 +532,12 @@ void RootWindow::SetFocusWhenShown(bool focused) {
   host_->SetFocusWhenShown(focused);
 }
 
+bool RootWindow::GrabSnapshot(const gfx::Rect& snapshot_bounds,
+                              std::vector<unsigned char>* png_representation) {
+  DCHECK(bounds().Contains(snapshot_bounds));
+  return host_->GrabSnapshot(snapshot_bounds, png_representation);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // RootWindow, Window overrides: