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"
 
 #if defined(SK_BUILD_FOR_WIN)
-    #include "../ports/SkMutex_win.h"
+    #include "../ports/SkMutex_win.h"  // IWYU pragma: export
 #else
-    #include "../ports/SkMutex_pthread.h"
+    #include "../ports/SkMutex_pthread.h"  // IWYU pragma: export
 #endif
 
 class SkAutoMutexAcquire : SkNoncopyable {
 public:
     explicit SkAutoMutexAcquire(SkBaseMutex& mutex) : fMutex(&mutex) {
         SkASSERT(fMutex != NULL);
         mutex.acquire();
     }
 
     explicit SkAutoMutexAcquire(SkBaseMutex* mutex) : fMutex(mutex) {
@@ skipping 23 matching lines @@
         fMutex->assertHeld();
     }
 
 private:
     SkBaseMutex* fMutex;
 };
 #define SkAutoMutexAcquire(...) SK_REQUIRE_LOCAL_VAR(SkAutoMutexAcquire)
 
 
 #endif//SkMutex_DEFINED