Remove SkThread.h, include SkMutex.h or SkAtomics.h as appropriate.

SkThread.h doesn't do anything anymore execept include those two,
and thankfully, it doesn't seem to be mentioned outside Skia.

No public API changes.
TBR=reed@google.com

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/c50acf2321d7a934c80d754e9cbe936dfb8eb4cc

Review URL: https://codereview.chromium.org/1215393002
diff --git a/src/lazy/SkDiscardableMemoryPool.cpp b/src/lazy/SkDiscardableMemoryPool.cpp
index 5b9d87f..70072c6 100644
--- a/src/lazy/SkDiscardableMemoryPool.cpp
+++ b/src/lazy/SkDiscardableMemoryPool.cpp
@@ -9,8 +9,8 @@
 #include "SkDiscardableMemoryPool.h"
 #include "SkImageGenerator.h"
 #include "SkLazyPtr.h"
+#include "SkMutex.h"
 #include "SkTInternalLList.h"
-#include "SkThread.h"
 
 // Note:
 // A PoolDiscardableMemory is memory that is counted in a pool.