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This brings us closer to the most-recent UX request that
I've seen:

- muting displays the previous volume level in a disabled
  state rather than displaying 0%
- volume down while muted drops the level to 0%
- volume up while muted unmutes and raises the level to 25%
  if it was 0% or restores the previous level otherwise

The requested behavior is actually to swell to the louder
volume over three seconds in the volume-up-while-muted case
instead of doing it instantly, but the mixer code doesn't
support that at present, so I'm punting on it for now.

BUG=chromium-os:8473,chromium-os:13618
TEST=manual

Review URL: http://codereview.chromium.org/7566018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@95873 0039d316-1c4b-4281-b951-d872f2087c98