GitLab

- Improved UX of the dev tools.
    a/ "Load unpacked", "Pack" and "Update now" buttons have been moved to the top
    b/ The header is now sticky like in chrome://extensions
    c/ The list of extensions/apps is centered when the window is expanded

BUG=242747
BUG=242743

Review URL: https://chromiumcodereview.appspot.com/15389003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202759 0039d316-1c4b-4281-b951-d872f2087c98