Fix #8661: Always allow WebKit to load popup URLs before switching tabs

[kylehickinson]

There was a race condition on older devices that where WebKit would not load the popup URL before Brave attempted to select the tab. Selecting the tab would then attempt to restore it using the tab's current URL which would be about:blank since the web view had not made any requests yet.

Summary of Changes

This pull request fixes #8661

Submitter Checklist:

• [x] Unit Tests are updated to cover new or changed functionality
• [x] User-facing strings use NSLocalizableString()
• [x] New or updated UI has been tested across:
  • [x] Light & dark mode
  • [x] Different size classes (iPhone, landscape, iPad)
  • [x] Different dynamic type sizes

Reviewer Checklist:

• [ ] Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• [ ] Necessary security reviews have taken place.
• [ ] Adequate unit test coverage exists to prevent regressions.
• [ ] Adequate test plan exists for QA to validate (if applicable).
• [ ] Issue and pull request is assigned to a milestone (should happen at merge time).