Cleanup: Remove unneeded forward declarations in ppapi.

ppapi/proxy/host_dispatcher.h

Index: ppapi/proxy/host_dispatcher.h
===================================================================
--- ppapi/proxy/host_dispatcher.h	(revision 109476)
+++ ppapi/proxy/host_dispatcher.h	(working copy)
@@ -18,22 +18,12 @@
 struct PPB_Proxy_Private;
 struct PPB_Var_Deprecated;
 
-namespace base {
-class WaitableEvent;
-}
-
-namespace IPC {
-class SyncChannel;
-}
-
 namespace ppapi {
 
 struct Preferences;
 
 namespace proxy {
 
-class VarSerialization;
-
 class PPAPI_PROXY_EXPORT HostDispatcher : public Dispatcher {
  public:
   // Constructor for the renderer side.
@@ -124,7 +114,7 @@
 // since that's what most callers have.
 class ScopedModuleReference {
  public:
-  ScopedModuleReference(Dispatcher* dispatcher);
+  explicit ScopedModuleReference(Dispatcher* dispatcher);
   ~ScopedModuleReference();
 
  private: