This also cleans up a few minor glitches in the extension texts,
and renames the EGL extension for consistency.

It incidentally fixes a bug in our EGL init where we were checking
the wrong client versions for KHR_create_context.

It also implements a new feature for tests which allow them to defer
Context creation until the test body. This allows tests to check for
EGL extension available before trying to create a context with certain
extensions.

BUG=angleproject:1635

Change-Id: I9311991332c357e36214082b16f2a4a57bfa8865
Reviewed-on: https://chromium-review.googlesource.com/450920
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>