Invalidate continuation blocks if outline changes.

If you have an inline element that has been broken apart into continuations
then we need to make sure we invalidate any blocks that were inside the
continuation if the outline changes.

BUG=396225

Review URL: https://codereview.chromium.org/437623002

git-svn-id: svn://svn.chromium.org/blink/trunk@179756 bbb929c8-8fbe-4397-9dbb-9b2b20218538