Comment fixes to Vector.

Actions missed feedback from https://chromiumcodereview.appspot.com/23601026/

R=eseidel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23990007

git-svn-id: svn://svn.chromium.org/blink/trunk@157843 bbb929c8-8fbe-4397-9dbb-9b2b20218538