It doesn't need to use a traverser, it's simpler to just iterate over
all statements in the global scope.

BUG=angleproject:1966
TEST=angle_unittests, WebGL conformance tests

Change-Id: I11200f72842db86be2bcdd11934262da183cc3b4
Reviewed-on: https://chromium-review.googlesource.com/504727
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>