These methods are safe to call while the page is still open, or when it is
already closed. Works in remotely connected browser as well.
Also makes video.path() to throw for remotely connected browser.
Under the hood migrated Download and Video to use the common Artifact object.
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.
