Fixed blank tab is shown after attaching split view

[simonhong]

fix brave/brave-browser#41852

This PR depends on #24478

When split view transfers to another browser,
its web contents should be cleared before attached to target browser.
It's done by SplitView::UpdateContentsWebViewVisual().

So far, we posted it to next turn when we break tile.
If we break a tile by unsplit, it doesn't cause any issue because
any other secondary web view doesn't host that secondary web contents.

However, if it's attached to another browser window, it's another story.
When we detach a split view from browser A and attach to browser B,
browser A breaks a tile after detached.
And then, they are attached to browser B and we make them as a split view.
If we don't clear secondary web contents after break the tile synchronously,
browser A and browser B's secondary web view has same web contents till posed `UpdateContentsWebViewVisual().
This could cause unexpected behavior like above issue.
So, secondary web view should be cleared synchronously after breaking a tile.

Screen.Recording.2024-12-12.143054.mp4

Resolves

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

SplitViewBrowserTest.BreakingTileMakesSecondaryWebViewHidden

See the linked issue for manul test