Remove unnecessary includes in src/pdf/

src/pdf/SkPDFBitmap.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #ifndef SkPDFBitmap_DEFINED
 #define SkPDFBitmap_DEFINED
 
-#include "SkPDFTypes.h"
+#include "SkRefCnt.h"
 
 class SkImage;
+class SkPixelSerializer;
+class SkPDFObject;
 
 /**
  * SkPDFBitmap wraps a SkImage and serializes it as an image Xobject.
  * It is designed to use a minimal amout of memory, aside from refing
  * the image, and its emitObject() does not cache any data.
  */
 sk_sp<SkPDFObject> SkPDFCreateBitmapObject(sk_sp<SkImage>,
                                            SkPixelSerializer*);
 
 #endif  // SkPDFBitmap_DEFINED