Fix mixed up comments, remove unnecessary type conversions, clarify
variable names and improve formatting in a few places.

TEST=angle_unittests, WebGL conformance tests
BUG=angleproject:913

Change-Id: Ice8fe3682d8e97f42747752302a1fba116132df4
Reviewed-on: https://chromium-review.googlesource.com/266843Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>