This reduces unnecessary memory allocations and conversions between
different objects containing the same data.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I87316509ab1cd6d36756ff6af7fa2b5c5a76a8ea
Reviewed-on: https://chromium-review.googlesource.com/827134Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>