Add instance counter of RenderObject, and expose it via WebLeakDetector.

This CL might affect rendering performance.  This CL should be reverted
if we have performance regression.

BUG=400588

Review URL: https://codereview.chromium.org/443503002

git-svn-id: svn://svn.chromium.org/blink/trunk@179559 bbb929c8-8fbe-4397-9dbb-9b2b20218538