fix broken clang build

R=mtklein@google.com

Review URL: https://codereview.chromium.org/23724014

git-svn-id: http://skia.googlecode.com/svn/trunk/src@11084 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/utils/SkCanvasStack.h b/utils/SkCanvasStack.h
index 2137d30..5311118 100644
--- a/utils/SkCanvasStack.h
+++ b/utils/SkCanvasStack.h
@@ -26,8 +26,8 @@
      * 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;