Also move the ANGLE modifications to eglext.h to a separate file
so we can more easily update and maintain the official header.

This is in preparation for adding the new program cache control
extension.

BUG=angleproject:1897

Change-Id: I3546e0c82f6536381301f62a115b42d580747a08
Reviewed-on: https://chromium-review.googlesource.com/559917
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>