chromeos: disables OAuth2Test.MergeSession as it is flaky

This test was disabled on linux, but it doesn't run on linux, rather
it runs on chromeos.

BUG=1249687
TEST=test only change

Change-Id: I094dc3455dee5b9b9ae30a3373f426d9f55e2dc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3163437
Commit-Queue: Scott Violet <sky@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Scott Violet <sky@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Owners-Override: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#921813}