[AudioPlayer] Ensure the current track is in the viewport after the current is changed

BUG=none
TEST=manually tested
R=hirono@chromium.org

Review URL: https://codereview.chromium.org/141773007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250065 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250065