Add methods to WebTestingSupport to detect memory leaks at content_shell

Added a functions to clean up and count DOM objects. This is intended to be used for leak detection. At the layout test, about:blank is loaded before and after one test, and by comparing the numbers of objects at about:blank, we can detect a leak.

https://docs.google.com/a/chromium.org/document/d/1sFAsZxeISKnbGdXoLZlB2tDZ8pvO102ePFQx6TX4X14/edit#

BUG=332630

Review URL: https://codereview.chromium.org/132913003

git-svn-id: svn://svn.chromium.org/blink/trunk@165626 bbb929c8-8fbe-4397-9dbb-9b2b20218538