In https://gerrit.wikimedia.org/r/#/c/379213/, basic autosave support implemented.
It connects the update events for the section nodes to the translation update events and do autosave. Section HTML content is extracted from the source and target
models. Some refactoring and utility method additions done for that.

Many sections throw errors while extracting content, for example sections with references. That need investigation and follow up fixes.

Success:

Change 379213 had a related patch set uploaded (by Catrope; owner: Santhosh):
[mediawiki/extensions/ContentTranslation@master] Autosave: On section content change, save the content.

https://gerrit.wikimedia.org/r/379213

Change 379213 merged by jenkins-bot:
[mediawiki/extensions/ContentTranslation@master] Autosave: On section content change, save the content

https://gerrit.wikimedia.org/r/379213

Auto-saving has been confirmed. After the patch for restoration [T176843] of a saved translation is merged, the complete workflow will need to be retested for a final confirmation.

In T176610#3631380, @santhosh wrote:

Many sections throw errors while extracting content, for example sections with references. That need investigation and follow up fixes.

This is fixed in https://gerrit.wikimedia.org/r/379213 itself

Checked in http://cx2-testing.wmflabs.org/index.php?
It correctly counts the time and displays it to a user: e.g. "Saved just now", "Saved 16 minutes ago".