Rename an argument named "register" to "reg".

This silences a warning in newer versions of clang that complains
about "register" being a deprecated keyword.

Bug: chromium:780692
Reviewed-on: https://chromium-review.googlesource.com/750362


Reviewed-by: Mark Mentovai <mark@chromium.org>

(cherry picked from commit 2d85d492aeb9eeb889885e84b3666d64407f2920)

Bug: http://b/111067277
Test: builds
Exempt-From-Owner-Approval: janitorial
Change-Id: I01d310dc397433a472e272f30d65813138f02cd5