Calculate the signal level on the media stream local audio track. And the signal level will be pass to libjingle after the new libjingle interface is implemented by chrome.

BUG=264611
TEST=content_unittests, nothing breaks.
R=tommi@chromium.org

Review URL: https://codereview.chromium.org/178223013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254759 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254759