Don't use `register`.

This causes a build failure with GCC and C++14. Fix by just not using
the register keyword.

"Guideline: Never write register. It's exactly as meaningful as
whitespace."
http://www.drdobbs.com/keywords-that-arent-or-comments-by-anoth/184403859

Change-Id: I275735c55e97e613e1704c1b896793740db8f404