Resubmit 'Enable the volume slider in Windows Ash'.

Previous 2 submissions were causing the Vista waterfall builder to fail
with uninitialized COM errors.  This change addresses that by
initializing COM on Vista+, not just Win7+.

TBRing all reviewers since this change has already been reviewed.

BUG=227247
TEST=Ran ash_unittests on a Vista VM
     * All tests failed without my change.
     * All tests passed with my change.
TBR=henrika,sky,jennyz

Review URL: https://codereview.chromium.org/197643003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256608 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256608