Only call createTabRequested if window is ready

[bsclifton]

referralPage loads quickly from server and code was previously calling createTabRequested well before window was ready.
This would cause the partner page to load first (before about:welcome).

Fixes #14220

Auditors: @NejcZdovc

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. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

See #14220

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[NejcZdovc]

++ looks good to me

[NejcZdovc]

master aa00fce
0.23 7b0a0e2
0.22 655c06b