Record layout rects on every layout.

Store them in a debug info class suitable for sending to
the compositor thread for use in UIs like about:tracing.

BUG=314945

Review URL: https://codereview.chromium.org/61823005

git-svn-id: svn://svn.chromium.org/blink/trunk@162715 bbb929c8-8fbe-4397-9dbb-9b2b20218538