Make PPAPI headers compile with C compilers (gcc on Linux & Mac and MSVS on W...

ppapi/cpp/core.cc

 // Copyright (c) 2010 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/cpp/core.h"
 
+#include "ppapi/cpp/common.h"
 #include "ppapi/cpp/completion_callback.h"
 
 namespace pp {
 
 // This function is implemented in the .cc file to avoid including completion
 // callback all over the project.
 void Core::CallOnMainThread(int32_t delay_in_milliseconds,
                             const CompletionCallback& callback,
                             int32_t result) {
   return interface_->CallOnMainThread(delay_in_milliseconds,
                                       callback.pp_completion_callback(),
                                       result);
 }
 
 bool Core::IsMainThread() {
-  return interface_->IsMainThread();
+  return PPBoolToBool(interface_->IsMainThread());
 }
 
 }  // namespace pp