The patch adds a function in MatchOutputCodeTest to get the position
of the first occurrence of an expression in the translated output code.
This can help design more complicated tests like a test which checks
for the order of simple expressions.

BUG=angleproject:2062
TEST=angle_unittests

Change-Id: I1249d4762c247848c4eec64ecb8c1357b5e8d40a
Reviewed-on: https://chromium-review.googlesource.com/563659Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>