Clean up a few includes, introduce iwyu.

include/core/SkMutex.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 SkMutex_DEFINED
 #define SkMutex_DEFINED
 
 // This file is not part of the public Skia API.
 #include "SkTypes.h"
 
+// IWYU pragma: begin_exports
 #if defined(SK_BUILD_FOR_WIN)
     #include "../ports/SkMutex_win.h"
 #else
     #include "../ports/SkMutex_pthread.h"
 #endif
+// IWYU pragma: end_exports
 
 template <typename Lock>
 class SkAutoTAcquire : SkNoncopyable {
 public:
     explicit SkAutoTAcquire(Lock& mutex) : fMutex(&mutex) {
         SkASSERT(fMutex != NULL);
         mutex.acquire();
     }
 
     explicit SkAutoTAcquire(Lock* mutex) : fMutex(mutex) {
@@ skipping 26 matching lines @@
 private:
     Lock* fMutex;
 };
 
 typedef SkAutoTAcquire<SkBaseMutex> SkAutoMutexAcquire;
 
 #define SkAutoMutexAcquire(...) SK_REQUIRE_LOCAL_VAR(SkAutoMutexAcquire)
 
 
 #endif//SkMutex_DEFINED