chore: introduce Page.viewportSizeChanged event

[dgozman]

Drive-by: small cleanup between viewportSize() and emulatedSize() on the server.

References #35987.

[github-actions]

Test results for "tests 1"

1 failed
❌ [webkit-library] › library/browsercontext-device.spec.ts:45:5 › device › should scroll to click @webkit-ubuntu-22.04-node18

5 flaky

⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:986:7 › cli codegen › should not throw csp directive violation errors @firefox-ubuntu-22.04-node18
⚠️ [webkit-library] › library/ignorehttpserrors.spec.ts:30:3 › should isolate contexts @webkit-ubuntu-22.04-node18
⚠️ [webkit-library] › library/video.spec.ts:475:5 › screencast › should scale frames down to the requested size @webkit-ubuntu-22.04-node18
⚠️ [webkit-page] › page/page-screenshot.spec.ts:345:5 › page screenshot › should work while navigating @webkit-ubuntu-22.04-node18
⚠️ [playwright-test] › ui-mode-test-watch.spec.ts:145:5 › should watch all @windows-latest-node18-1

39233 passed, 804 skipped
✔️✔️✔️

Merge workflow run.