Was
I420AlphaToARGB_Premult (8861 ms)
I420AlphaToARGB_Opt (7119 ms)
Now
I420AlphaToABGR_Premult (2840 ms)
I420AlphaToARGB_Opt (484 ms)

C function switched to 1 step.
Was
I420AlphaToARGB_Premult (8862 ms)
I420AlphaToABGR_Opt (6718 ms)

Now
I420AlphaToARGB_Premult (8706 ms)
I420AlphaToARGB_Opt (6541 ms)

R=harryjin@google.com
BUG=libyuv:496, libyuv:473

Review URL: https://codereview.chromium.org/1359183003 .