Clean up a few includes, introduce iwyu.

include/core/SkTypes.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTypes_DEFINED
 #define SkTypes_DEFINED
 
+// IWYU pragma: begin_exports
 #include "SkPreConfig.h"
 #include "SkUserConfig.h"
 #include "SkPostConfig.h"
+#include <stddef.h>
 #include <stdint.h>
 #include <sys/types.h>
 
 #if defined(SK_ARM_HAS_NEON)
     #include <arm_neon.h>
 #elif SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
     #include <immintrin.h>
 #endif
+// IWYU pragma: end_exports
+
+#include <stdlib.h>
+#include <string.h>
 
 /** \file SkTypes.h
 */
 
 /** See SkGraphics::GetVersion() to retrieve these at runtime
  */
 #define SKIA_VERSION_MAJOR  1
 #define SKIA_VERSION_MINOR  0
 #define SKIA_VERSION_PATCH  0
 
@@ skipping 643 matching lines @@
 private:
     void*       fPtr;
     size_t      fSize;  // can be larger than the requested size (see kReuse)
     uint32_t    fStorage[(kSize + 3) >> 2];
 };
 // Can't guard the constructor because it's a template class.
 
 #endif /* C++ */
 
 #endif