Remove ResetAreAllSessionsMergedForTesting

Doesn't seem to be used.

Change-Id: I5de5df2e89513feb18709ea2d30baa362a97e824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566912
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651062}
2 files changed