Add asserts for shared mutex.

src/core/SkSharedMutex.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSharedLock_DEFINED
 #define SkSharedLock_DEFINED
 
@@ skipping 10 matching lines @@
 class SkSharedMutex {
 public:
     SkSharedMutex();
     ~SkSharedMutex();
     // Acquire lock for exclusive use.
     void acquire();
 
     // Release lock for exclusive use.
     void release();
 
+    // Fail if exclusive is not held.
+#ifdef SK_DEBUG
+    void assertHeld() const;
+#else
+    void assertHeld() const {}
+#endif
+
     // Acquire lock for shared use.
     void acquireShared();
 
     // Release lock for shared use.
     void releaseShared();
 
+    // Fail if shared lock not held.
+#ifdef SK_DEBUG
+    void assertHeldShared() const;
+#else
+    void assertHeldShared() const {}
+#endif
+
 private:
     SkAtomic<int32_t> fQueueCounts;
     SkSemaphore fSharedQueue;
     SkSemaphore fExclusiveQueue;
 };
 
+
 #endif // SkSharedLock_DEFINED