Unify the values of "source" and "page" as used in FeatureCallout.sys.mjs:
- Explicitly pass in a value for "page" when instantiating a Feature Callout and use this for the value of "page" when sending Feature Callout telemetry and as the "source" when making calls to `sendTriggerMessage`. This avoids the risk of including non-about: page URLs or PDF file extensions in our telemetry.
- Set the value of "page" in an HTML data attribute that can be accessed for use in about:welcome telemetry for Spotlight and Feature Callouts.
- Update references to the page value previously used as the page/source for telemetry from `about:firefoxview` Feature Callouts from "firefoxview" to "about:firefoxview"
- Pass the token "chrome" when creating a callout from the browser chrome and update references to the source in PDF.js messages' targeting
- Update the page value expected in automated tests as needed
Differential Revision: https://phabricator.services.mozilla.com/D165910
04e1d3b419