Fix #1373: Set dummy tab id for private mode browsing.

[iccub]

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• My patch or PR title has a standard commit message that looks like Fix #123: This fixes the shattered coffee cup! (or No Bug: <message> if no relevant ticket)
• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New files have MPL-2.0 license header.

Test Plan:

Screenshots:

Reviewer Checklist:

• PR is linked to an issue via Zenhub.
• Issues are assigned to at least one epic.
• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable)

[iccub]

Please test yubikey functionality before merging, I tested regular browsing activity, moving tabs, restoring..

[jumde]

Please test yubikey functionality before merging, I tested regular browsing activity, moving tabs, restoring..

Works with YubiKey

[iccub]

@jhreis do we merge to development or to a hotfix branch?