Fix undefined behavior.

src/core/SkSharedMutex.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSharedMutex.h"
 
 #include "SkAtomics.h"
 #include "SkSemaphore.h"
 #include "SkTypes.h"
 
-
 #if defined(THREAD_SANITIZER)
 
 /* Report that a lock has been created at address "lock". */
 #define ANNOTATE_RWLOCK_CREATE(lock) \
     AnnotateRWLockCreate(__FILE__, __LINE__, lock)
 
 /* Report that the lock at address "lock" is about to be destroyed. */
 #define ANNOTATE_RWLOCK_DESTROY(lock) \
     AnnotateRWLockDestroy(__FILE__, __LINE__, lock)
 
@@ skipping 136 matching lines @@
         fSharedQueue.wait();
     }
     ANNOTATE_RWLOCK_ACQUIRED(this, 0);
    
 }
 
 void SkSharedMutex::releaseShared() {
     ANNOTATE_RWLOCK_RELEASED(this, 0);
 
     // Decrement the shared count.
-    int32_t oldQueueCounts = fQueueCounts.fetch_add(-1 << kSharedOffset,
+    int32_t oldQueueCounts = fQueueCounts.fetch_add(~0U << kSharedOffset,
                                                     sk_memory_order_release);
 
     // If shared count is going to zero (because the old count == 1) and there are exclusive
     // waiters, then run a single exclusive waiter.
     if (((oldQueueCounts & kSharedMask) >> kSharedOffset) == 1
         && (oldQueueCounts & kWaitingExclusiveMask) > 0) {
         fExclusiveQueue.signal();
     }
 }