When command line --libyuv_cpu_info is used the individual tests
used to need to set the cpumask.  This CL moves that to the init
for each test class so the individual tests dont need to set it.

TBR=kjellander@chromium.org
BUG=libyuv:720
TEST=LibYUVBaseTest.TestCpuHas

Change-Id: I6ae180388debf6cf76be6df5b81cfffeb35ee2eb
Reviewed-on: https://chromium-review.googlesource.com/662367Reviewed-by: Cheng Wang <wangcheng@google.com>