Resolve FIXME in Document: if style invalidation needs to be updated, then there must be
a style resolver, since there is no longer a style resolver timer and the only code that
could have registered invalidation dirty bits is inside the style resolver.

BUG=

Review URL: https://codereview.chromium.org/171793004

git-svn-id: svn://svn.chromium.org/blink/trunk@167461 bbb929c8-8fbe-4397-9dbb-9b2b20218538