Clean up a few includes, introduce iwyu.

include/core/SkAtomics.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 SkAtomics_DEFINED
 #define SkAtomics_DEFINED
 
@@ skipping 50 matching lines @@
     bool compare_exchange(T* expected, const T& desired,
                           sk_memory_order success = sk_memory_order_seq_cst,
                           sk_memory_order failure = sk_memory_order_seq_cst) {
         return sk_atomic_compare_exchange(&fVal, expected, desired, success, failure);
     }
 private:
     T fVal;
 };
 
 #if defined(_MSC_VER)
-    #include "../ports/SkAtomics_std.h"
+    #include "../ports/SkAtomics_std.h"  // IWYU pragma: export
 #elif !defined(SK_BUILD_FOR_IOS) && defined(__ATOMIC_RELAXED)
-    #include "../ports/SkAtomics_atomic.h"
+    #include "../ports/SkAtomics_atomic.h"  // IWYU pragma: export
 #else
-    #include "../ports/SkAtomics_sync.h"
+    #include "../ports/SkAtomics_sync.h"  // IWYU pragma: export
 #endif
 
 // From here down we have shims for our old atomics API, to be weaned off of.
 // We use the default sequentially-consistent memory order to make things simple
 // and to match the practical reality of our old _sync and _win implementations.
 
 inline int32_t sk_atomic_inc(int32_t* ptr)            { return sk_atomic_fetch_add(ptr, +1); }
 inline int32_t sk_atomic_dec(int32_t* ptr)            { return sk_atomic_fetch_add(ptr, -1); }
 inline int32_t sk_atomic_add(int32_t* ptr, int32_t v) { return sk_atomic_fetch_add(ptr,  v); }
 
@@ skipping 21 matching lines @@
 template <typename T>
 T sk_acquire_load(T* ptr) { return sk_atomic_load(ptr, sk_memory_order_acquire); }
 
 template <typename T>
 void sk_release_store(T* ptr, T val) { sk_atomic_store(ptr, val, sk_memory_order_release); }
 
 inline void sk_membar_acquire__after_atomic_dec() {}
 inline void sk_membar_acquire__after_atomic_conditional_inc() {}
 
 #endif//SkAtomics_DEFINED