In RendererGL, the EXT_debug_marker functions were calling the
KHR_debug entry points, now they fall back only when EXT_debug_marker is
missing.

Separated the ContextImpl methods for the two extensions.

BUG=781164

Change-Id: I615b5965b705e55eb730ebefa6e27e0ee6d86c31
Reviewed-on: https://chromium-review.googlesource.com/786337Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>