GitLab

Add a listener on the ExtensionList object for when the count of extensions
moves from 0 to 1, or from 1 to 0 - i.e., when we should show or hide the
list.

BUG=480110

Review URL: https://codereview.chromium.org/1105683003

Cr-Commit-Position: refs/heads/master@{#327201}