Refactors ANGLE extensions context calls into a common format for future
autogeneration. Any work that occurred in the entry point has been moved into
its own context member function.

Bug:angleproject:2263
Change-Id: I91cb3aca6a8c135ed3ae80a723d6cf16b54aeda6
Reviewed-on: https://chromium-review.googlesource.com/990809Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>