Implement PlatformDispatcher.requestViewFocusChange on web.

[tugorez]

Implement PlatformDispatcher.requestViewFocusChange on web.

Relevant Issues are:

• Design doc: https://flutter.dev/go/focus-management
• Focus in web multiview: https://github.com/flutter/flutter/issues/137443
• Platform dispatcher changes: https://github.com/flutter/engine/pull/49841

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the CLA.
• [x] All existing and new tests are passing.

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