Bug 282980 – [WebDriver][BiDi] Implement log events buffering

• Component: WebDriver
• Status: NEW

[Lauro Moura]

https://w3c.github.io/webdriver-bidi/#module-log

> A BiDi Session has a log event buffer which is a map from navigable id to a list of log events for that context that have not been emitted. User agents may impose a maximum size on this buffer, subject to the condition that if events A and B happen in the same context with A occurring before B, and both are added to the buffer, the entry for B must not be removed before the entry for A.

From `log.entryAdded` event definition: https://w3c.github.io/webdriver-bidi/#event-log-entryAdded

> 16. If event is enabled with session, "log.entryAdded" and related navigables, emit an event with session and body.

>     Otherwise, buffer a log event with session, related browsing contexts, and body.

Storing events in buffer spec: https://w3c.github.io/webdriver-bidi/#buffer-a-log-event

And in the end of `log.entryAdded` spec we have the definition of emitting these buffered events, which happen at the "remote end subscribe steps", with priority 10. That is, when the user subscribes to `log.entryAdded`, the remote end "flushes" the buffered events in order back to the client.

[Radar WebKit Bug Importer]

<rdar://problem/140188887>