GitLab

Use ExtensionRegistryObserver in Uber
since ExtensionRegistry replaces chrome::extension notifications.

BUG=411568
R=estade@chromium.org
TEST=browser_test --gtest_filter=UberUIBrowserTest.HistoryOverride

Review URL: https://codereview.chromium.org/1012773010

Cr-Commit-Position: refs/heads/master@{#322281}