Clean up a few includes, introduce iwyu.

include/core/SkMutex.h

Index: include/core/SkMutex.h
diff --git a/include/core/SkMutex.h b/include/core/SkMutex.h
index ea7e81726b88c05828f35753c877b129eab2a15d..56b1a2724e0ff2d4f881b1d995b6fdb6e74fcfbf 100644
--- a/include/core/SkMutex.h
+++ b/include/core/SkMutex.h
@@ -1,3 +1,10 @@
+/*
+ * 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
 
@@ -5,9 +12,9 @@
 #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 {