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.
Sometimes I see "cannot call emit on the undefined" error on the bots.
This change adds some more logging, so we could potentially identify where
the issue comes from.
Root index.js is only used for local development, so
assuming dev mode there is fine. This way we do not have
to worry about calling setUnderTest early enough.
