SK_DECLARE_STATIC_MUTEX -> static SkMutex

There's no need to use a macro to declare static SkMutexes any more
(and there's likewise no need to restrict them to global scope).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1948193002

Review-Url: https://codereview.chromium.org/1948193002
diff --git a/src/lazy/SkDiscardableMemoryPool.cpp b/src/lazy/SkDiscardableMemoryPool.cpp
index d6753de..da7db52 100644
--- a/src/lazy/SkDiscardableMemoryPool.cpp
+++ b/src/lazy/SkDiscardableMemoryPool.cpp
@@ -244,7 +244,7 @@
     return new DiscardableMemoryPool(size, mutex);
 }
 
-SK_DECLARE_STATIC_MUTEX(gMutex);
+static SkMutex gMutex;
 
 SkDiscardableMemoryPool* SkGetGlobalDiscardableMemoryPool() {
     static SkOnce once;