Handle offline cases for sync

[darkdh]

fix brave/brave-browser#2139
fix brave/brave-browser#2137
fix brave/brave-browser#2278

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:

Unit test

• npm run test -- brave_unit_tests --filter=BraveSyncServiceTest.*

Manual tests

• STR in Device creates a new sync chain when using existing code words  brave-browser#2139, Sync chain creation fails after system comes online brave-browser#2137 and Sync chain is deleted when relaunching browser in offline status brave-browser#2278

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

[AlexeyBarabash]

++

[darkdh]

master: af8537b
0.58.x: 043bac7
0.57.x: b653a19

[bbondy]

Reverted from 0.57.x due to no approval:
e87c177