This adds two properties to the Tab object:
- isArticle indicates whether the document in the tab is likely able to be
rendered in reader mode.
- isInReaderMode indicates if the document in the tab is being rendered in
reader mode.
It also adds a toggleReaderMode() which toggles a tab into and out of reader mode.
There is also a new case in which tabs.onUpdated will fire. When the isArticle
status of a tab changes, an onUpdated event will fire with data {isArticle: boolean}.
MozReview-Commit-ID: AaAQ0V5qm2Z
4ac6021faa