fix SK_LEGACY_ENCODE_BITMAP missing cast

BUG=skia:
TBR=
NOTRY=True
NOTREECHECKS=True

Review URL: https://codereview.chromium.org/815223002

src/core/SkPicture.cpp

@@ -472,7 +472,7 @@ public:
         // Required by signature of EncodeBitmap.
         size_t unused;
         SkBitmap bm;
-        bm.installPixels(info, pixels, rowBytes);
+        bm.installPixels(info, const_cast<void*>(pixels), rowBytes);
         return fEncoder(&unused, bm);
     }