Add a debug assert that layout is successful

This patch adds an assert for debug builds that layout completes with
all nodes being laid out.

BUG=266213
NOTRY=true
R=inferno@chromium.org

Review URL: https://codereview.chromium.org/21329002

git-svn-id: svn://svn.chromium.org/blink/trunk@155280 bbb929c8-8fbe-4397-9dbb-9b2b20218538