Add asserts for shared mutex.

BUG=skia:

Review URL: https://codereview.chromium.org/1285973003
diff --git a/src/core/SkSharedMutex.h b/src/core/SkSharedMutex.h
index a3535dc..f343004 100644
--- a/src/core/SkSharedMutex.h
+++ b/src/core/SkSharedMutex.h
@@ -28,16 +28,31 @@
     // 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