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/633945Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>