Reduce number of atomic references to cpu_info by making
InitCpuFlags call MaskCpuFlags and return the same value.

BUG=libyuv:641
TEST=libyuv_unittests pass

Change-Id: I5dfff8f7a10671bc8ef3ec0ed6f302791e752faa
Reviewed-on: https://chromium-review.googlesource.com/514145
Commit-Queue: Frank Barchard <fbarchard@google.com>
Reviewed-by: Cheng Wang <wangcheng@google.com>