random / rand is slow and impacts performance testing.
Although its only called to clear a frame once, a typical profile shows
it high in the overall profile, when doing 1000 frames for a benchmark.

95.10%  libyuv_unittest  libyuv_unittest      [.] YUY2ToARGBRow_SSSE3
 2.01%  libyuv_unittest  libc-2.19.so         [.] __random_r
 1.13%  libyuv_unittest  libc-2.19.so         [.] __random

Replace random is a faster version for unittests.

set LIBYUV_WIDTH=1280
set LIBYUV_HEIGHT=720
set LIBYUV_REPEAT=999
set LIBYUV_FLAGS=-1
out\release\libyuv_unittest --gtest_filter=*YUY2ToARGB*  | findms

Was
libyuvTest.YUY2ToARGB_Opt (497 ms)

Now
libyuvTest.YUY2ToARGB_Opt (454 ms)

R=harryjin@google.com
BUG=none

Review URL: https://codereview.chromium.org/1361813002 .