[GPU] Allow kMulRGBByAlpha_InConfigFlag and kSwapRAndB_InConfigFlag to both be set
[GPU] Allow kMulRGBByAlpha_InConfigFlag and kSwapRAndB_InConfigFlag to both be set

Review URL: http://codereview.appspot.com/5433049/



csulmone/skia,csulmone/skia,csulmone/skia,csulmone/skia