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Code changes:
- Disabling onboarding content for ChromeOS. Promotions for CrOS are out-of-scope for this launch.
- Ensuring that preference-specified tabs do not appear in new windows when a tabbed browser already exists for the profile.

Test changes:
- Removed 12 outdated tests
- Fixed or updated 9 tests

Details of affected tests, and rationale, can be found in this doc: https://docs.google.com/a/google.com/document/d/1PjAemEEFrl0gIDVQfnr1oAnHkb6DcV47HlOyyo7ICZI/edit?usp=sharing

BUG=608875,314819,313856

Review-Url: https://codereview.chromium.org/2475913003
Cr-Original-Commit-Position: refs/heads/master@{#442487}
Committed: https://chromium.googlesource.com/chromium/src/+/a52322134679a349366417e04e502bc8ce1f68e4
Review-Url: https://codereview.chromium.org/2475913003
Cr-Commit-Position: refs/heads/master@{#442712}