Use observer for an X message pump to intercept keyevents for volume control.

chrome/browser/chromeos/system_key_event_listener.h

Index: chrome/browser/chromeos/system_key_event_listener.h
diff --git a/chrome/browser/chromeos/system_key_event_listener.h b/chrome/browser/chromeos/system_key_event_listener.h
index 2a6ebf6c97192b6ca411ec9d2a2426453cc7eb1d..7ba32d5530404ae0da68d243df5b5b970cf510b2 100644
--- a/chrome/browser/chromeos/system_key_event_listener.h
+++ b/chrome/browser/chromeos/system_key_event_listener.h
@@ -9,8 +9,11 @@
 #include <gdk/gdk.h>
 
 #include "base/memory/singleton.h"
+#include "base/message_loop.h"
 #include "chrome/browser/chromeos/wm_message_listener.h"
 
+typedef union _XEvent XEvent;
+
 namespace chromeos {
 
 class AudioHandler;
@@ -22,7 +25,8 @@ class AudioHandler;
 // TODO(davej): Remove WmMessageListener::Observer once volume key handling has
 // been removed from the window manager since those keys take precedence.
 
-class SystemKeyEventListener : public WmMessageListener::Observer {
+class SystemKeyEventListener : public WmMessageListener::Observer,
+                               public MessageLoopForUI::Observer {
  public:
   static SystemKeyEventListener* GetInstance();
 
@@ -55,6 +59,15 @@ class SystemKeyEventListener : public WmMessageListener::Observer {
   void OnVolumeDown();
   void OnVolumeUp();
 
+  // MessageLoopForUI::Observer overrides.
+  virtual void WillProcessEvent(GdkEvent* event) OVERRIDE {}
+  virtual void DidProcessEvent(GdkEvent* event) OVERRIDE {}
+  virtual bool WillProcessXEvent(XEvent* xevent)
+#if defined(TOUCH_UI)
+    OVERRIDE
+#endif
+    ;
+
   int32 key_volume_mute_;
   int32 key_volume_down_;
   int32 key_volume_up_;