Add support for labeling of GL objects for debug purposes.

Tests: DebugTest.ObjectLabelsEXT*
Bug: angleproject:5337
Change-Id: I792a27c69395ed62da70c3f619bef71776fba432
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2568711Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>