Proposal: Include documentId in Tab object

[fregante]

Context

The documentId has use cases throughout the WebExtensions APIs, and more is to come. However, the only ways to retrieve it seems to be by pinging a content script or using the WebNavigation permission (getFrame, onCommitted).

The WebNavigation permission has a permissions warning that many extension publishers will like to avoid. It's also safer for the user if the extension doesn't need it. The developer might also not have a content script available for pinging.

Proposal

The Tab object already includes information on the tab's main frame like URL, pendingURL, favicon, etc. I propose also including the documentId and pendingDocumentId. That way it can be retrieved through tabs.get or tabs.query.

Originally posted by @newRoland in #495

[fregante]

I'll add: having the documentId readily available when querying tabs can help ensure that follow-up actions are towards a specific document only, like executeScript or sendMessage.

Also, I assume that this piece of information, like tabId and frameId, can be made available without any specific permissions.