chromeos: Update volume key behavior.

chromeos: Update volume key behavior.

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

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=95873

[Daniel Erat, 2011-08-06 00:24:36]

ping

[DaveMoore, 2011-08-07 16:44:17]

lgtm