Software volume adjustment by changing samples.

      BUG=16500
TEST=play youtube in different window.  play another video with video tag and mute sound and make sure it does not affect youtube.

Review URL: http://codereview.chromium.org/159092

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@21091 0039d316-1c4b-4281-b951-d872f2087c98