Structures with names ending in "_#" such as "_0" could conflict
with the internally rewritten scoped structures. Fix this by using
a prepending rule instead of appending.

Also includes a test, and fixes a WebGL test in Firefox. (Chrome is
not affected because of the variable hashing step.)

BUG=angle:618

Change-Id: I3d441f1de268b6d7e74a0834b43e889b7bfe578c
Reviewed-on: https://chromium-review.googlesource.com/201468Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>