Use atomic subtract in shared mutex.

src/core/SkSharedMutex.cpp

Index: src/core/SkSharedMutex.cpp
diff --git a/src/core/SkSharedMutex.cpp b/src/core/SkSharedMutex.cpp
index 4cf63120676e658f43b1b354c22d27b08eb7d549..05e434c35b8f259b7e645e5b4119ec2d2b8d1f58 100644
--- a/src/core/SkSharedMutex.cpp
+++ b/src/core/SkSharedMutex.cpp
@@ -184,7 +184,7 @@ void SkSharedMutex::releaseShared() {
     ANNOTATE_RWLOCK_RELEASED(this, 0);
 
     // Decrement the shared count.
-    int32_t oldQueueCounts = fQueueCounts.fetch_add(~0U << kSharedOffset,
+    int32_t oldQueueCounts = fQueueCounts.fetch_sub(1 << kSharedOffset,
                                                     sk_memory_order_release);
 
     // If shared count is going to zero (because the old count == 1) and there are exclusive