Clean up a few includes, introduce iwyu.

src/ports/SkDiscardableMemory_none.cpp

Index: src/ports/SkDiscardableMemory_none.cpp
diff --git a/src/ports/SkDiscardableMemory_none.cpp b/src/ports/SkDiscardableMemory_none.cpp
index 51c3164dfa6e90ea3b29f31a29461bc23bf49bcf..be6002838bd33d331b000821576cc6624a102688 100644
--- a/src/ports/SkDiscardableMemory_none.cpp
+++ b/src/ports/SkDiscardableMemory_none.cpp
@@ -5,8 +5,11 @@
  * found in the LICENSE file.
  */
 
+#include "SkDiscardableMemory.h"
+
+#include <stddef.h>
+
 #include "SkDiscardableMemoryPool.h"
-#include "SkTypes.h"
 
 SkDiscardableMemory* SkDiscardableMemory::Create(size_t bytes) {
     return SkGetGlobalDiscardableMemoryPool()->create(bytes);