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 08b220c3968c9b62f97495db15c8cdf1661992dc..eb5e29b80a7e1c0ad06ec8d4f292711aeae08599 100644
--- a/include/core/SkMutex.h
+++ b/include/core/SkMutex.h
@@ -11,11 +11,13 @@
 // This file is not part of the public Skia API.
 #include "SkTypes.h"
 
+// IWYU pragma: begin_exports
 #if defined(SK_BUILD_FOR_WIN)
     #include "../ports/SkMutex_win.h"
 #else
     #include "../ports/SkMutex_pthread.h"
 #endif
+// IWYU pragma: end_exports
 
 template <typename Lock>
 class SkAutoTAcquire : SkNoncopyable {