Use a document selector pattern only in multi-root

[DonJayamanne]

For #5333

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• [n/a] Has sufficient logging.
• [n/a] Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• [n/a] Test plan is updated as appropriate
• [n/a] package-lock.json has been regenerated by running npm install (if dependencies have changed)
• [n/a] The wiki is updated with any design decisions/details.

[ericsnowcurrently]

Aside from a couple of minor comments, LGTM.

Also, just be be sure I understood the PR, the only real change is on line 151, right? Everything else is just factoring out the private getDocumentSelector() method (which I approve, BTW :) ).