[Extensions] Rework inline installation observation

      Instead of observing through the WebstoreAPI, observe directly in the TabHelper.
This is a great deal less code, more direct, and also fixes a lifetime issue
with the TabHelper being deleted before the inline installation completes.

BUG=613949

Review-Url: https://codereview.chromium.org/2103663002
Cr-Commit-Position: refs/heads/master@{#403188}