In preparation for constant folding fixes.

BUG=chromium:637050

Change-Id: I9ea49ce96b34c6ac3d2f0478b8fc6732c59e28be
Reviewed-on: https://chromium-review.googlesource.com/373741Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>