Add a unified resource tracker shared between the proxy and the impl.

ppapi/proxy/plugin_dispatcher.h

Index: ppapi/proxy/plugin_dispatcher.h
diff --git a/ppapi/proxy/plugin_dispatcher.h b/ppapi/proxy/plugin_dispatcher.h
index ae6da6045e0e04daf303ecad183f61b6e3491f09..b2fbeec549dcab28665844c5befd351b1d64ddef 100644
--- a/ppapi/proxy/plugin_dispatcher.h
+++ b/ppapi/proxy/plugin_dispatcher.h
@@ -26,6 +26,7 @@ class WaitableEvent;
 
 namespace ppapi {
 struct Preferences;
+class Resource;
 }
 
 namespace pp {
@@ -84,6 +85,10 @@ class PluginDispatcher : public Dispatcher {
   // DidCreateInstance/DidDestroyInstance.
   static PluginDispatcher* GetForInstance(PP_Instance instance);
 
+  // Same as GetForInstance but retrieves the instance from the given resource
+  // object as a convenience. Returns NULL on failure.
+  static PluginDispatcher* GetForResource(const ppapi::Resource* resource);
+
   static const void* GetInterfaceFromDispatcher(const char* interface);
 
   // You must call this function before anything else. Returns true on success.