DevTools: add a warning mark to Layout events that have nested autosize

... also, generalize warnings support in Timeline.

BUG=

Review URL: https://codereview.chromium.org/29043002

git-svn-id: svn://svn.chromium.org/blink/trunk@160337 bbb929c8-8fbe-4397-9dbb-9b2b20218538