PPAPI/NaCl: Move handle extraction code to ppapi/proxy

ppapi/proxy/ppb_audio_proxy.h

Index: ppapi/proxy/ppb_audio_proxy.h
diff --git a/ppapi/proxy/ppb_audio_proxy.h b/ppapi/proxy/ppb_audio_proxy.h
index 2f85a83d4f79b5d319a3b27ebf63ef9cee3d7f32..757bdeb8686a381756458f58ae806bdd410e59bc 100644
--- a/ppapi/proxy/ppb_audio_proxy.h
+++ b/ppapi/proxy/ppb_audio_proxy.h
@@ -17,7 +17,6 @@
 #include "ppapi/c/ppb_audio_config.h"
 #include "ppapi/proxy/interface_proxy.h"
 #include "ppapi/proxy/proxy_completion_callback_factory.h"
-#include "ppapi/proxy/serialized_structs.h"
 #include "ppapi/utility/completion_callback_factory.h"
 
 namespace ppapi {
@@ -26,6 +25,8 @@ class HostResource;
 
 namespace proxy {
 
+class SerializedHandle;
+
 class PPB_Audio_Proxy : public InterfaceProxy {
  public:
   PPB_Audio_Proxy(Dispatcher* dispatcher);