An error will be generated either way if the extension symbols are
used incorrectly since each use of an extension function or variable
checks whether the extension is enabled.

We now also track extension in unmangled built-in names, so that
redefining built-ins of extensions that are not enabled can be
supported.

This includes refactoring the shader extension tests to share a common
helper class ShaderExtensionTest.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I9cc5e9bd62fa07796e69256a6a9a493531a62446
Reviewed-on: https://chromium-review.googlesource.com/926526
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>