add const to encodePixels pixel parameter

src/core/SkPicture.cpp

Index: src/core/SkPicture.cpp
diff --git a/src/core/SkPicture.cpp b/src/core/SkPicture.cpp
index 3296264be8ab76ac666f0adf91f106bc5536dc7b..921fbdf532a3e9340e12c30f3404dd374e0b58cb 100644
--- a/src/core/SkPicture.cpp
+++ b/src/core/SkPicture.cpp
@@ -465,9 +465,9 @@ public:
         SkASSERT(fEncoder);
     }
 
-    virtual bool onUseEncodedData(const void*, size_t) SK_OVERRIDE { return true; }
+    bool onUseEncodedData(const void*, size_t) SK_OVERRIDE { return true; }
 
-    virtual SkData* onEncodePixels(const SkImageInfo& info, void* pixels,
+    SkData* onEncodePixels(const SkImageInfo& info, const void* pixels,
                                    size_t rowBytes) SK_OVERRIDE {
         // Required by signature of EncodeBitmap.
         size_t unused;