Assert we are not repainting during Layout.

With the --enable-repaint-after-layout flag we should not be triggering a repaint
while we are inside layout.

BUG=320139

Review URL: https://codereview.chromium.org/132343002

git-svn-id: svn://svn.chromium.org/blink/trunk@166259 bbb929c8-8fbe-4397-9dbb-9b2b20218538