Added flag that enables system sounds whether ChromeVox is enabled or not.

BUG=322820
TEST=manual tests on Lumpy

Review URL: https://codereview.chromium.org/105573008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244844 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244844