Fix bookmark hanger dialog alignment bug

[luixxiul]

Closes #9040
Follow-up to #8985 (comment)

Auditors:

Test Plan 1:

1. Click the star icon on the URL bar
2. Make sure the bookmark hanger appears under the star icon, not at the center of the browser window

Test Plan 2:

1. "Add bookmark" on the bookmark toolbar
2. Make sure the bookmark dialog appears at the center of the browser window

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).

Reviewer Checklist:

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

[cezaraugusto]

++