The expression limit validation check needs to be in front
of other tree traversal to prevent stack overflows. Also,
call depths of sufficient size (80k+) could overflow the
depth check itself, necessitating an upper bound on initial
tree traversal.

This fixes crashes in the WebGL long-expressions bug test.

BUG=angle:584

Change-Id: Ib48294bf77a5923d230f237fbd63a36a5662e317
Reviewed-on: https://chromium-review.googlesource.com/191931Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>