Partial fix for https://github.com/microsoft/playwright/issues/6319
After this fix, the following scenario won't leak and the context state
(cookies, storage, etc) can be reused by the new page sessions:
```js
for (let i = 0; i < 1000; ++i) {
const page = await context.newPage();
await page.goto('...');
await page.close('...');
}
```
Instead of requiring all frames in the subtree to receive a particular
event, we rely on the browser's definition of load and DOMContentLoaded.
This changes logic in a few edge cases:
- Some browsers do not emit load event upon window.stop() at all.
- DOMContentLoaded does not wait for subframes, so they might not be
ready when passing `{ waitUntil: 'domcontentloaded' }`.
`networkidle` preserves the old logic.
