Currently, we always throw from FrameSession._stopScreencast
when not running with video, and immediately catch it in
CRPage.didClose (thanks to debugger to point that).
Overall, we have code prepared for start/stop API, which
we never did, so it makes sense to simplify code a bit,
and throw if something goes wrong.
- This leaves just `recordVideos` and `videoSize` options on the context.
- Videos are saved to `artifactsPath`. We also save their ids to trace.
- `context.close()` waits for the processed videos.
This is a large rework of selectors:
- Each BrowserContext now has a separate Selectors instance that has its own registrations.
Most of them share a single sharedSelectors instance, but contexts created for a connected
browser have their own instance.
- Connected browser now gets a RemoteBrowser object that encapsulates Selectors and Browser.
This Selectors object is registered with the api selectors.
- Public selectors.register api iterates over all registered Selectors channels
and registers in each of them.
- createSelector testing method migrated to ElementHandle._createSelectorForTest.
