The `OnTraceBufferUsageAvailable` callback creates V8 handles via
`Dictionary::CreateEmpty()` before `promise.Resolve()` enters its
`SettleScope` (which provides a `HandleScope`). When the callback
fires asynchronously from a Mojo response (i.e. when a trace session
is active), there is no `HandleScope` on the stack, causing a fatal
V8 error: "Cannot create a handle without a HandleScope".

Add an explicit `v8::HandleScope` at the top of the callback, matching
the pattern used by the other contentTracing APIs which resolve their
promises through `SettleScope` or the static `ResolvePromise` helper.

Made-with: Cursor

Co-authored-by: Alexey Kozy <alexey@anysphere.co>