This CL just adds comments to the synchroinze*Attribute methods on Element, no change in behavior.

BUG=322239

Review URL: https://codereview.chromium.org/102963006

git-svn-id: svn://svn.chromium.org/blink/trunk@164034 bbb929c8-8fbe-4397-9dbb-9b2b20218538