Covers both GL_KHR_debug and GL_EXT_debug_marker.

Debug markers are used to specify events or hierarchically categorize a
set of commands within the command buffer.  When debugging, this allows
for quicker navigation to the draw calls of interest, and otherwise
provides context to debug output.

Bug: angleproject:2853
Change-Id: Id65e11fc877d9e70b6fd0fae7f0bbbcb1164bf10
Reviewed-on: https://chromium-review.googlesource.com/c/1403956
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>