add peekPixels to SkCanvas and SkSurface

src/core/SkPictureRecord.h

Index: src/core/SkPictureRecord.h
diff --git a/src/core/SkPictureRecord.h b/src/core/SkPictureRecord.h
index 0df120b31a121b54f5681e6bed5dc3e75fc5cab3..eaa5e730081c95b1c4b9d7576875d340a585a35d 100644
--- a/src/core/SkPictureRecord.h
+++ b/src/core/SkPictureRecord.h
@@ -221,6 +221,9 @@ public:
 
 protected:
     virtual SkSurface* onNewSurface(const SkImageInfo&) SK_OVERRIDE;
+    const void* onPeekPixels(SkImageInfo*, size_t*) SK_OVERRIDE {
+        return NULL;
+    }
 
     // Return fontmetrics.fTop,fBottom in topbot[0,1], after they have been
     // tweaked by paint.computeFastBounds().