Fix assert that BackgroundDownloader's timer os deleted on wrong thread

The timer and two interface pointers are leaked if a download is
active when the browser is shutting down.

Releasing these resources on the file thread is impractical to do at
this time due to the browser shutdown sequence. The file thread is
terminated before the dtor of this class is called and there is no
obvious way to hook up the shutdown of the browser to terminate a
download in progress and dispose of these resources.

BUG=341083

Review URL: https://codereview.chromium.org/156983005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249662 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249662
Remove expand logic from notifications

Via experimentation we determined that expand/collapse is not the best
behavior for the notification center.  This removes the code that was
behind a flag to enable the feature.  Future patch will update
the default layout so that collapse/expand is unnecessary.

r=dimich@chromium.org,oshima@chromium.org
BUG=276004

Review URL: https://codereview.chromium.org/156713004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249663 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249663