Cleanup: Remove unneeded forward declarations in ppapi.

ppapi/tests/test_video_decoder.h

Index: ppapi/tests/test_video_decoder.h
===================================================================
--- ppapi/tests/test_video_decoder.h	(revision 109579)
+++ ppapi/tests/test_video_decoder.h	(working copy)
@@ -5,15 +5,16 @@
 #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();