Don't iterate children in recalcStyle if no style changed

We were iterating through all the children doing lots of wasted work
when no style really changed.

With a really basic benchmark of 1000 <div>'s inside a parent div and
then setting the border of that parent div to trigger a recalc that
doesn't effect the children this shows a 18-25% reduction in the time
spent in recalcStyle. The real reduction is about 0.01ms which isn't
huge, but it should reduce the amount of cache thrashing we do and makes
us scale much better.

Unfortunately we still iterate all the children inside
RenderObject::propagateStyleToAnonymousChildren so changes like this are
linear in the number of children. Eventually we should figure out a way
to fix that too.

For reference Firefox can do this in constant time.

BUG=315271

Review URL: https://codereview.chromium.org/59033004

git-svn-id: svn://svn.chromium.org/blink/trunk@161394 bbb929c8-8fbe-4397-9dbb-9b2b20218538