Introduce XGetImage() for GrabWindowSnapshot() in ChromeOS.

ui/aura/root_window_host_mac.mm

Index: ui/aura/root_window_host_mac.mm
diff --git a/ui/aura/root_window_host_mac.mm b/ui/aura/root_window_host_mac.mm
index fb3caaaaf905b393050df5bc2ce6dae28f9b50f2..4c9f69aba826386f60a96cc4c5b06759810f300d 100644
--- a/ui/aura/root_window_host_mac.mm
+++ b/ui/aura/root_window_host_mac.mm
@@ -45,6 +45,9 @@ class RootWindowHostMac : public RootWindowHost,
   virtual void MoveCursorTo(const gfx::Point& location) OVERRIDE;
   virtual bool ConfineCursorToRootWindow() OVERRIDE;
   virtual void UnConfineCursor() OVERRIDE;
+  virtual bool GrabSnapshot(
+      const gfx::Rect& snapshot_bounds,
+      std::vector<unsigned char>* png_representation) OVERRIDE;
 
   // RootWindowHostMacDelegate:
   virtual void SendEvent(const base::NativeEvent& native_event) OVERRIDE;
@@ -169,6 +172,13 @@ bool RootWindowHostMac::ConfineCursorToRootWindow() {
 void RootWindowHostMac::UnConfineCursor() {
 }
 
+bool RootWindowHostMac::GrabSnapshot(
+    const gfx::Rect& snapshot_bounds,
+    std::vector<unsigned char>* png_representation) {
+  NOTIMPLEMENTED();
+  return false;
+}
+
 void RootWindowHostMac::SendEvent(const base::NativeEvent& native_event) {
   ui::EventType type = ui::EventTypeFromNative(native_event);
   switch (type) {