Add assertHeld() to SkMutex.

src/core/SkScaledImageCache.cpp

Index: src/core/SkScaledImageCache.cpp
diff --git a/src/core/SkScaledImageCache.cpp b/src/core/SkScaledImageCache.cpp
index a6bffd899fe66938a91fccf1dc90660d5d7e9a51..b69b949b394e44f1c5c62f9bd908472e6f41203a 100644
--- a/src/core/SkScaledImageCache.cpp
+++ b/src/core/SkScaledImageCache.cpp
@@ -675,6 +675,7 @@ static void cleanup_gScaledImageCache() { SkDELETE(gScaledImageCache); }
 /** Must hold gMutex when calling. */
 static SkScaledImageCache* get_cache() {
     // gMutex is always held when this is called, so we don't need to be fancy in here.
+    gMutex.assertHeld();
     if (NULL == gScaledImageCache) {
 #ifdef SK_USE_DISCARDABLE_SCALEDIMAGECACHE
         gScaledImageCache = SkNEW_ARGS(SkScaledImageCache, (SkDiscardableMemory::Create));