Import default search engine

[bsclifton]

Fixes brave/brave-browser#2421
Fixes brave/brave-browser#2415

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

1. Create a new Muon profile
   • open about:preferences#search
   • set default search engine to Bing
   • set Use DuckDuckGo by default for search in Private Tabs to true
   • set Use DuckDuckGo by default for search in Private Tabs with Tor to false
2. Launch Brave Core
3. Visit chrome://settings/importData
4. Pick Brave (old) from the drop down and import (doesn't matter which items you pick)
5. When import is finished, visit chrome://settings/searchEngines
6. Verify default search engine is Bing
7. Try a search in this window; verify it uses Bing
8. Open a new Private Window and verify Search with DuckDuckGo is checked
9. Try a search in this window; verify it uses DDG
10. Close Private Windows + Open a NEW Private Window with Tor
11. DDG logo and text should not show
12. Try a search in this window; verify it uses Bing

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[bsclifton]

Reduced scope of this PR, removed WIP label. Originally, I was going to put multiple fixes in here, but I think just these two are a great start.

I added a test plan and ran into issues when manually testing it. Basically, the private tab settings didn't import properly. Looking at this now...

[bsclifton]

OK private windows now properly respect the DDG true/false value

I found a problem with Tor windows that I think is outside of the scope of this change. This PR will properly set kUseAlternativeSearchEngineProvider (which gets inherited by guest window). However, the engine doesn't seem to respect that value:
https://github.com/brave/brave-core/blob/master/browser/search_engines/tor_window_search_engine_provider_service.cc#L51-L67

[bsclifton]

@mkarolin @darkdh ready for re-review 😄 👍

[darkdh]

lgtm, please clean up commits then I will approve

[bsclifton]

@darkdh commits squashed! 😄👌

[kjozwiak]

@bsclifton uplift request to 0.58.x approved after deliberating with @srirambv & @rebron 👍 Please ensure that the correct labels are added/removed and the associated issue is moved to the correct milestone.

[bsclifton]

master c77e76b
0.60.x bdd4dfd
0.59.x 5d5a411
0.58.x bcc8403

[bsclifton]

0.58.x broke after the above cherry-pick; fixed with #1090