This change is purely refactoring existing functionality to make it
easier to change variable identifier parsing.

BUG=angleproject:911
BUG=541551

Change-Id: I282fbb66d40cd71fa0f4804e4a8e893744cd6bfc
Reviewed-on: https://chromium-review.googlesource.com/309724Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>