Because of virtualized contexts, the debug groups are not always popped
in the same order that they are pushed leading to inconsistant state and
sometimes GL_STACK_OVERFLOW errors from the driver.

BUG=angleproject:2354

Change-Id: I0a001002149184dd882c3b17e8451bcd694608ba
Reviewed-on: https://chromium-review.googlesource.com/909794Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>