Don't recalc all styles when adding @keyframes, @import and @page

This applies to @import and @page rules as well.
There are probably other rule types which we could
be smarter about, but @keyframes is most common.

I wish I had a good way to test this.  I considered
writing an inspector-timeline test, or adding a
"style recalc count" to settings.internals.  I think
long-term we should consider exposing recalc information
as part of window.performance?  But for now there
doesn't seem to be a nice way to test this change.

Elliott and I discovered when writing/reviewing this
that adding @keyframes after an animation is first
created has no effect in WebKit/Blink.  It should!
I've filed: http://crbug.com/307760

BUG=307725

Review URL: https://codereview.chromium.org/26983004

git-svn-id: svn://svn.chromium.org/blink/trunk@160277 bbb929c8-8fbe-4397-9dbb-9b2b20218538