The old code was accidentally using letters offset by 10 when writing
out hex values >= 10. Now the letters a to f are used as they should.

This is one issue that changed shader output when ImmutableString was
introduced, so it is a potential cause for a regression detailed in
bug 824062, though this has not been verified.

BUG=chromium:824062
TEST=angle_unittests

Change-Id: Idb871dffba32a3ab20df0fe17b4b1a98ec00b7fa
Reviewed-on: https://chromium-review.googlesource.com/999480
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>