Prompt user to close Chrome before importing from a Chrome profile

[garrettr]

The UX and implementation are based on how Chrome's Firefox importer prompts the user to close Firefox before importing from a Firefox profile. ChromeProfileLock provides the same interface as FirefoxProfileLock, and the implementation is simpler because we are able to reuse Chromium's ProcessSingleton to check if Chrome is running at the time of import.

Resolves brave/brave-browser#103.

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).
• 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.

Test Plan

Manual testing

1. You will need a Chrome profile with some profile data saved in it.
2. Start Chrome in the background.
3. yarn start
4. Open the main menu (e.g. Brave on macOS) and choose Import Bookmarks and Settings...
5. Choose your test Chrome profile from the dropdown menu of browser profiles and click Import.
6. You should see an alert with the title "Close Chrome," the text "To finish importing, close all Chrome windows," and two buttons: Try again and Cancel.
7. Cancel should return you to the Settings page without importing anything from the Chrome profile.
8. Try again should return you to the same alert repeatedly, until the background Chrome process is quit.
9. Quit the background Chrome process, then click Try again.
10. Your Chrome profile data should be successfully imported.

Cross-platform testing

I have verified that this PR builds and passes the manual tests described above on macOS, Windows, and Linux.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions

[darkdh]

we should set lock_acquired_ to false here and notice once this singleton has been cleanup, it can't be used again. Just in case we will use Unlock else where other than destructor in the future and also have a correct behavior in unittest.
addressed in a44a433

[darkdh]

++