Const types make it possible to implement caching and
other optimizations.

BUG=492725

Change-Id: I64398bb9effcc909dd052a038acbb5ec0ca730e8
Reviewed-on: https://chromium-review.googlesource.com/281046Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Dmitry Skiba <dskiba@google.com>