Migrate Graphics2D to new design.

ppapi/host/ppapi_host.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ppapi/host/ppapi_host.h"
 
 #include "base/logging.h"
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/host/host_factory.h"
 #include "ppapi/host/host_message_context.h"
@@ skipping 159 matching lines @@
 
 void PpapiHost::OnHostMsgResourceDestroyed(PP_Resource resource) {
   ResourceMap::iterator found = resources_.find(resource);
   if (found == resources_.end()) {
     NOTREACHED();
     return;
   }
   resources_.erase(found);
 }
 
-ResourceHost* PpapiHost::GetResourceHost(PP_Resource resource) {
-  ResourceMap::iterator found = resources_.find(resource);
+ResourceHost* PpapiHost::GetResourceHost(PP_Resource resource) const {
+  ResourceMap::const_iterator found = resources_.find(resource);
   return found == resources_.end() ? NULL : found->second.get();
 }
 
 }  // namespace host
 }  // namespace ppapi