Add HandleInputEventAck message to allow out-of-process plugins to respond to user gestures.

ppapi/proxy/ppp_input_event_proxy.h

Index: ppapi/proxy/ppp_input_event_proxy.h
===================================================================
--- ppapi/proxy/ppp_input_event_proxy.h	(revision 125463)
+++ ppapi/proxy/ppp_input_event_proxy.h	(working copy)
@@ -6,6 +6,7 @@
 #define PPAPI_PROXY_PPP_INPUT_EVENT_PROXY_H_
 
 #include "ppapi/c/pp_instance.h"
+#include "ppapi/c/pp_time.h"
 #include "ppapi/c/ppp_input_event.h"
 #include "ppapi/proxy/interface_proxy.h"
 
@@ -33,6 +34,8 @@
                                      const ppapi::InputEventData& data,
                                      PP_Bool* result);
 
+  void HandleInputEventAck(PP_Instance instance, PP_TimeTicks timestamp);
+
   // When this proxy is in the plugin side, this value caches the interface
   // pointer so we don't have to retrieve it from the dispatcher each time.
   // In the host, this value is always NULL.