Use unsigned for size() and capacity() driven from HashTable.

This is a follow-up to HashTable API change in blink r156965.

R=cevans@chromium.org, abarth@chromium.org, tsepez@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/60883004

git-svn-id: svn://svn.chromium.org/blink/trunk@162356 bbb929c8-8fbe-4397-9dbb-9b2b20218538