Increased style sharing for elements with presentation attributes.

Increased style sharing for elements with presentation attributes.
<http://webkit.org/b/78199>

Reviewed by Antti Koivisto.

When determining whether two elements can share style, we can do a lot better.
Instead of comparing the attribute maps for exact equality, do a property-by-property
comparison of the attributeStyle() and the additionalAttributeStyle() (if any.)

This increases our style sharing hit rate and shaves 100ms off of each cycle on
Chromium's "Moz" page cycler test on my machine.

The function that compares attribute styles has O(n^2) runtime in the worst case,
where n is the number of properties in the styles. However, given the low number of
properties found in attribute styles, this should be fine, and it doesn't seem to
heat up in profiles.

* css/CSSStyleSelector.cpp:
(WebCore::attributeStylesEqual):
(WebCore::CSSStyleSelector::canShareStyleWithElement):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107173 268f45cc-cd09-0410-ab3c-d52691b4dbfc