fix(ext/web): honor PerformanceObserver buffered flag (#34748)
## What
When `PerformanceObserver.observe()` is called with the single-type form
and `options.buffered = true`, replay the global performance entry
buffer
into the observer's buffer and queue the callback. Previously, a
freshly-
created observer with `buffered: true` never saw entries that were
recorded before `observe()` was called.
## Why
[Performance Timeline
§observe()](https://w3c.github.io/performance-timeline/#dom-performanceobserver-observe)
says:
> 7. If options's buffered flag is set:
> 1. Let queue be the result of running get entries on relevant global
object's performance entry buffer with type and null.
> 2. Append the entries in queue to observer's observer buffer.
> 3. Queue the PerformanceObserver task.
Our implementation only delivered newly-emitted entries to observers, so
the buffered flag was a no-op.
## WPT subtests now passing
- `user-timing/buffered-flag.any.html` — *PerformanceObserver with
buffered flag sees previous marks*
- `user-timing/buffered-flag.any.html` — *PerformanceObserver with
buffered flag sees previous measures*
- `user-timing/buffered-flag.any.worker.html` — same two subtests in the
worker harness
Expectations file `tests/wpt/runner/expectations/user-timing.json`
flipped from `false` to `true` for both files.
## Root cause
`PerformanceObserver.prototype.observe` in `ext/web/15_performance.js`
was
missing the buffered-flag branch entirely.
