Clean up a few includes, introduce iwyu.

include/core/SkAtomics.h

Index: include/core/SkAtomics.h
diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index be94164227b789c43899cabd38c94d6fe8e3e050..7036e0035c243b9cf9ba9651fbc31384577e670f 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.h
@@ -68,11 +68,11 @@ private:
 };
 
 #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.