Adds support for GL_LINE_LOOP with primitive restart.

Bug: angleproject:3215
Change-Id: Ief1bdf15ef9b108dba025eaf4ce580bba54af623
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1649351Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>