Add assertHeld() to SkMutex.

src/core/SkPaint.cpp

Index: src/core/SkPaint.cpp
diff --git a/src/core/SkPaint.cpp b/src/core/SkPaint.cpp
index 78d947020b20442ac8bafcc63f14d752bf2f0c83..569df4de87204a3666ccc3a3873e1f789ab980ca 100644
--- a/src/core/SkPaint.cpp
+++ b/src/core/SkPaint.cpp
@@ -1737,6 +1737,7 @@ static SkScalar gDeviceGamma = SK_ScalarMin;
  * the returned SkMaskGamma pointer is refed or forgotten.
  */
 static const SkMaskGamma& cachedMaskGamma(SkScalar contrast, SkScalar paintGamma, SkScalar deviceGamma) {
+    gMaskGammaCacheMutex.assertHeld();
     if (0 == contrast && SK_Scalar1 == paintGamma && SK_Scalar1 == deviceGamma) {
         if (NULL == gLinearMaskGamma) {
             gLinearMaskGamma = SkNEW(SkMaskGamma);