Add extension string to registry_xml.py and autogen files.

Bug: angleproject:3231
Change-Id: I57d7f8d8e9292f32b2ae461c539296eb63ce26d4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2432270
Commit-Queue: Jeff Vigil <j.vigil@samsung.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>