Fix Chrome OS Account Manager Settings page UI tests

These tests currently fail because they depend on Account Manager being
enabled. Please check the attached bug for context.

Fix this by enabling Account Manager via feature flags in these tests.

Bug: 977739
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: Ibbcdca7f34864c8b253a6054e47ce3720dafd2b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1680255
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672970}