Add asserts for shared mutex.

src/core/SkSharedMutex.h

Index: src/core/SkSharedMutex.h
diff --git a/src/core/SkSharedMutex.h b/src/core/SkSharedMutex.h
index a3535dca430aad26b26ab33dc5ef4018609e96df..f3430040e38b51ee05ddb9d2915db44a30ff6ec3 100644
--- a/src/core/SkSharedMutex.h
+++ b/src/core/SkSharedMutex.h
@@ -28,16 +28,31 @@ public:
     // 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