cpu_id cleanup.  no functional change.
cpu_id cleanup.  no functional change.

remove old comment about initialize to zero.
remove ifdef and replace with macro defined to zero.

BUG=None
TEST=try bots
R=kjellander@chromium.org

Review URL: https://codereview.chromium.org/2425623004 .

lemenkov/libyuv,yarrcc/libyuv-ios,yarrcc/libyuv-ios,lemenkov/libyuv,lemenkov/libyuv,yarrcc/libyuv-ios,lemenkov/libyuv,yarrcc/libyuv-ios