Instead of returning false when a check succeeds in ParseContext,
return true. This is more intuitive and in line with conventions used
elsewhere inside ANGLE.

Also includes some minor other cleanup in ParseContext, like improved
variable names and code structure especially when checking array
element properties. This will make introducing arrays of arrays easier
in the future.

BUG=angleproject:911
TEST=angle_unittests

Change-Id: I68233c01ccfbfef9529341af588f615efc2b503a
Reviewed-on: https://chromium-review.googlesource.com/371238Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>