The maxLayer() method was reading out of range for 2D arrays.

BUG=644846,614178
BUG=angleproject:1493

Change-Id: I7cc4773d1ee9ff2d0a18cb0a8e916cf3687446db
Reviewed-on: https://chromium-review.googlesource.com/382332
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>