Use "unsigned" instead of "size_t" in Deque to save space on 64-bit.

This mirrors changes we've made to Vector.
Deque is not widely used, however, so we don't expect magic.

Review URL: https://chromiumcodereview.appspot.com/23629047

git-svn-id: svn://svn.chromium.org/blink/trunk@157819 bbb929c8-8fbe-4397-9dbb-9b2b20218538