Refactor GLSL array length method parsing This prepares for accepting arbitrary expressions as the "this" node of the array length method. BUG=angleproject:2142 TEST=angle_unittests Change-Id: I728adb6e76d2779dedbabfaeec7d096872e0d00d Reviewed-on: https://chromium-review.googlesource.com/633945 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>