Cleanup: Remove unneeded forward declarations in ppapi.

ppapi/tests/test_video_decoder.h

 // Copyright (c) 2011 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.
 
 #ifndef PPAPI_TESTS_TEST_VIDEO_DECODER_H_
 #define PPAPI_TESTS_TEST_VIDEO_DECODER_H_
 
+#include <string>
+
 #include "ppapi/c/pp_stdint.h"
 #include "ppapi/tests/test_case.h"
 
-struct PPB_Var;
 struct PPB_VideoDecoder_Dev;
 
 class TestVideoDecoder : public TestCase {
  public:
-  TestVideoDecoder(TestingInstance* instance) : TestCase(instance) {}
+  explicit TestVideoDecoder(TestingInstance* instance) : TestCase(instance) {}
 
   // TestCase implementation.
   virtual bool Init();
   virtual void RunTest();
 
   void QuitMessageLoop();
 
  private:
   std::string TestCreateFailure();
 
   // Used by the tests that access the C API directly.
   const PPB_VideoDecoder_Dev* video_decoder_interface_;
 };
 
 #endif  // PPAPI_TESTS_TEST_VIDEO_DECODER_H_