Avoid copying a hash table bucket when inserting causes a rehash

From WebKit r155571 by Anders Carlsson <andersca@apple.com>

Allow rehashing to track a hash bucket and return the new pointer to it, thus avoiding having to do a copy of a hash table bucket as well as an extra hash lookup.

Review URL: https://codereview.chromium.org/23622031

git-svn-id: svn://svn.chromium.org/blink/trunk@158626 bbb929c8-8fbe-4397-9dbb-9b2b20218538