Add a release assert when painting text without a layout.

This patch adds a release assert when painting text without a layout.
Until the larger issue in crbug.com/264211 is finished this will
prevent bad things from happening with a partially laid out tree.

BUG=264211

Review URL: https://chromiumcodereview.appspot.com/20664003

git-svn-id: svn://svn.chromium.org/blink/trunk@155381 bbb929c8-8fbe-4397-9dbb-9b2b20218538