When the ANGLE end2end tests change back-ends, the null driver must
install the correct global DebugAnnotator, so that the DebugAnnotator
from another driver is not used.

Bug: chromium:1135860
Change-Id: I11aa386995283aa214df1faa62392dc6a6cbef0d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2457126Reviewed-by: Ian Elliott <ianelliott@google.com>
Commit-Queue: Ian Elliott <ianelliott@google.com>