DevTools: [CSS] fix crash if rule source datas do not align with CSS model

CSS agent does not handle changes through CSSOM well (see crbug.com/178410).
As a result, internal mapping between css rule model and their source datas
is not always maintained, in this case resulting in out-of-bounds bug.

BUG=373508

Review URL: https://codereview.chromium.org/286143005

git-svn-id: svn://svn.chromium.org/blink/trunk@174163 bbb929c8-8fbe-4397-9dbb-9b2b20218538