API change - I420AlphaToARGB takes flag indicating if RGB should be
premultiplied by alpha.

This version implements an efficient SSSE3 version for Windows.
C version done in 2 steps.

Was
libyuvTest.I420AlphaToARGB_Any (1136 ms)
libyuvTest.I420AlphaToARGB_Unaligned (1210 ms)
libyuvTest.I420AlphaToARGB_Invert (966 ms)
libyuvTest.I420AlphaToARGB_Opt (1031 ms)
libyuvTest.I420AlphaToABGR_Any (1020 ms)
libyuvTest.I420AlphaToABGR_Unaligned (1359 ms)
libyuvTest.I420AlphaToABGR_Invert (1082 ms)
libyuvTest.I420AlphaToABGR_Opt (986 ms)

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

Review URL: https://codereview.chromium.org/1367093002 .