Avoid style sharing with mis-matched descendant selectors

Avoid style sharing with mis-matched descendant selectors

Switching to an LRU for finding elements for style sharing resulted in
elements sharing that don't share a direct grandparent. Because of this, 
the code to check that elements can share style didn't take descendant
selectors into account. This patch breaks descendant selectors out and
resolves them as part of checking that elements' styles can be shared.

BUG=305502

[leviw_travelin_and_unemployed, 2013-10-20 03:01:58]

I haven't written a description yet, but I wrote a different solution for the
descendant selector failure case. Ideally, I want to cache the result.

[esprehn, 2013-10-21 19:19:07]

I don't think this will work, you're making us match every rule in the entire
sheet against each element in the middle of style sharing.

Storing a Vector that's the size of the sheet also sounds bad. Almost every rule
contains a descendant selector in practice.

https://codereview.chromium.org/29633003/diff/1/Source/core/css/RuleFeature.h
File Source/core/css/RuleFeature.h (right):

https://codereview.chromium.org/29633003/diff/1/Source/core/css/RuleFeature.h...
Source/core/css/RuleFeature.h:83: Vector<RuleFeature> descendantRules;
Most rules are descendant rules, this Vector is going to be nearly the size of
the stylesheet itself.

https://codereview.chromium.org/29633003/diff/1/Source/core/css/resolver/Shar...
File Source/core/css/resolver/SharedStyleFinder.cpp (right):

https://codereview.chromium.org/29633003/diff/1/Source/core/css/resolver/Shar...
Source/core/css/resolver/SharedStyleFinder.cpp:273: if
(!candidateMatchesDescendantRules(context, element))
The hotest part of style sharing is matching the rulesets at the end. Matching
against almost the entire sheet (since nearly every rule is a descendant rule)
is going to be very very slow.

https://codereview.chromium.org/29633003/diff/1/Source/core/css/resolver/Shar...
Source/core/css/resolver/SharedStyleFinder.cpp:330: return
collector.matchedResult().ranges == candidateCollector.matchedResult().ranges;
Matching rules like this is really expensive.