fix broken clang build

src/utils/SkCanvasStack.h

Index: src/utils/SkCanvasStack.h
diff --git a/src/utils/SkCanvasStack.h b/src/utils/SkCanvasStack.h
index 2137d308b805610b80a5ad22cccd72121847812b..531111827038791fcf152d88fbfb619e4b7b5974 100644
--- a/src/utils/SkCanvasStack.h
+++ b/src/utils/SkCanvasStack.h
@@ -26,8 +26,8 @@ public:
      * can share most of the other implementation of NWay we override those
      * methods to be no-ops.
      */
-    virtual void addCanvas(SkCanvas*) SK_OVERRIDE { SkASSERT(!"Invalid Op"); }
-    virtual void removeCanvas(SkCanvas*) SK_OVERRIDE { SkASSERT(!"Invalid Op"); }
+    virtual void addCanvas(SkCanvas*) SK_OVERRIDE { SkDEBUGFAIL("Invalid Op"); }
+    virtual void removeCanvas(SkCanvas*) SK_OVERRIDE { SkDEBUGFAIL("Invalid Op"); }
 
     virtual void setMatrix(const SkMatrix& matrix) SK_OVERRIDE;
     virtual bool clipRect(const SkRect&, SkRegion::Op, bool) SK_OVERRIDE;