Refactor to send iframe resize messages directly from layout thread to constellation
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#14682.
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r? @jdm
passing tests:
tests/wpt/mozilla/tests/css/matchMedia.html, tests/wpt/mozilla/tests/mozilla/window_resize_not_triggered_on_load.html, tests/wpt/mozilla/tests/mozilla/iframe/resize_after_load.html, tests/wpt/mozilla/tests/css/meta_viewport_resize.html
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