[Mac] User manager should show up as a standalone window.

BUG=324040
TEST=With the --new-profile-management flag on, start Chrome and click the avatar button. Select "View All Users". This should bring up the User Manager in a new, top-level window.

Review URL: https://codereview.chromium.org/102913002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241336 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241336
Remove the use of GetDefaultProfileOrOffTheRecord

BUG=322682
TEST=covered by unit tests

Review URL: https://codereview.chromium.org/106973014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241337 0039d316-1c4b-4281-b951-d872f2087c98
Use flexbox to center the plugin.

This allows us to use CSS instead of JS to do the centering, and means that
there's no need for "position: absolute", and so will which make measuring the
DOM easier when we come to implement scrolling for apps v2.

NOTRY=true

Review URL: https://codereview.chromium.org/114473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241338 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241338