Previously, the simplified-fullscreen-ui flag controlled whether to use
the new Views fullscreen notification bubble, or the old Cocoa one. Now
that this flag has been removed on Mac, the Cocoa bubble code is no
longer used. This change should have no visible effect.

BUG=610900

Review-Url: https://codereview.chromium.org/2001423002
Cr-Commit-Position: refs/heads/master@{#396393}