Add a layout test for counting the number of nodes that need layout.

Pull out the inspector code into a helper function on Frame so that
we can expose it to internals. We may want to expose the total
subtree size count as well, but this seems like a good first start.

Review URL: https://codereview.chromium.org/119963008

git-svn-id: svn://svn.chromium.org/blink/trunk@164565 bbb929c8-8fbe-4397-9dbb-9b2b20218538