Keep only one traverser for looking up symbol nodes by name instead
of having two largely identical ones.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I36e906258180e22b7b1353cab79d90266d99fa0e
Reviewed-on: https://chromium-review.googlesource.com/836895Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>