Add const where possible to AudioSyncReader member variables.

BUG=none
TEST=none
R=scherkus@chromium.org

Review URL: https://codereview.chromium.org/56293002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232503 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232503