Introduce XGetImage() for GrabWindowSnapshot() in ChromeOS.

ui/aura/root_window_host.h

Index: ui/aura/root_window_host.h
diff --git a/ui/aura/root_window_host.h b/ui/aura/root_window_host.h
index 47bba42b8e40aa74f785282931147f31a0d3d2f5..03647f99643e31b5798cd2be5fffd5b30a32c6ec 100644
--- a/ui/aura/root_window_host.h
+++ b/ui/aura/root_window_host.h
@@ -6,6 +6,8 @@
 #define UI_AURA_ROOT_WINDOW_HOST_H_
 #pragma once
 
+#include <vector>
+
 #include "base/message_loop.h"
 #include "ui/base/cursor/cursor.h"
 #include "ui/gfx/native_widget_types.h"
@@ -88,6 +90,11 @@ class RootWindowHost {
   // Sets if the window should be focused when shown.
   virtual void SetFocusWhenShown(bool focus_when_shown) = 0;
 
+  // Grabs the snapshot of the root window by using the platform-dependent APIs.
+  virtual bool GrabSnapshot(
+      std::vector<unsigned char>* png_representation,
+      const gfx::Rect& snapshot_bounds) = 0;
+
   // Posts |native_event| to the platform's event queue.
 #if !defined(OS_MACOSX)
   virtual void PostNativeEvent(const base::NativeEvent& native_event) = 0;