Revert of SK_DECLARE_STATIC_MUTEX -> static SkMutex

src/lazy/SkDiscardableMemoryPool.cpp

Index: src/lazy/SkDiscardableMemoryPool.cpp
diff --git a/src/lazy/SkDiscardableMemoryPool.cpp b/src/lazy/SkDiscardableMemoryPool.cpp
index da7db529583234ff4b2fbb13b23ef281a85a0e9b..d6753def9d2263e84bc8d156c0e317616b5d4025 100644
--- a/src/lazy/SkDiscardableMemoryPool.cpp
+++ b/src/lazy/SkDiscardableMemoryPool.cpp
@@ -244,7 +244,7 @@
     return new DiscardableMemoryPool(size, mutex);
 }
 
-static SkMutex gMutex;
+SK_DECLARE_STATIC_MUTEX(gMutex);
 
 SkDiscardableMemoryPool* SkGetGlobalDiscardableMemoryPool() {
     static SkOnce once;