https://bugs.webkit.org/show_bug.cgi?id=75508
        
Reviewed by Dave Hyatt.

It is a relatively common pattern to use inline stylesheets in document body where all rules are scoped using descendant selector

<style>
#foo {...}
#foo div {...}
#foo .bar {...}
</style>

When this pattern is used it is also common that the rules only apply to elements that come after the style element. 

When the set of active stylesheets changes we invalidate and recompute the entire document style. This is very expensive. We can 
detect the case above and avoid the style recalc.
        
On engadget.com, this patch cuts the time spent in style recalcs to roughly half. There are further savings from reduced 
relayouts. In total the engine CPU time used over the page load is reduced by ~10%.

* css/CSSStyleSelector.cpp:
(WebCore::CSSStyleSelector::CSSStyleSelector):
(WebCore::CSSStyleSelector::collectFeatures):
        
    Refactor feature collection from constructor to a separate function.
        
(WebCore::CSSStyleSelector::appendAuthorStylesheets):
        
    New function for non-destructively updating the style selector.
        
(WebCore::CSSStyleSelector::Features::clear):
        
    Clear the features for another collection.
            
(WebCore::CSSStyleSelector::determineStylesheetSelectorScopes):
        
    Find if all rules on a stylesheetare scoped to some set of ids and classes.
        
(WebCore::CSSStyleSelector::styleForDocument):
        
    Add optional font selector argument. We updated the correct base style font on style selector construction but that is no longer sufficient
    as font selector may be updated without reconstructing the style selector.
        
(WebCore::RuleSet::addRulesFromSheet):
        
    Invalidate the matched declaration cache in case of new font-face rules.
        
* css/CSSStyleSelector.h:
* css/SelectorChecker.cpp:
(WebCore::SelectorChecker::determineSelectorScopes):
        
    Find if all rules on a selector list are scoped to some set of ids and classes.
        
* css/SelectorChecker.h:
* dom/Document.cpp:
(WebCore::Document::Document):
(WebCore::Document::recalcStyle):
        
    Pass the font selector, if exists, to styleForDocument so we always have the base font up to date.
        
(WebCore::Document::combineCSSFeatureFlags):
(WebCore::Document::resetCSSFeatureFlags):
(WebCore::Document::createStyleSelector):

    Refactor css feature flag resetting to functions.
        
(WebCore::Document::removePendingSheet):
        
    Use new PendingStylesheetCompleted flag when new stylesheets arrive.

(WebCore::Document::styleSelectorChanged):

    Skip style recalc if it is not needed.

(WebCore::Document::collectActiveStylesheets):
        
    Refactor collecting stylesheets to a separate function.
        
(WebCore::Document::testAddedStylesheetRequiresStyleRecalc):
        
    Determine the scopes and use hasElementWithId/getElementsByClassName to figure out if any scoped elements currently exist in the tree.
        
(WebCore::Document::analyzeStylesheetChange):
        
    Figure out if we can update the style selector incrementally and if we can skip the style recalc.
        
(WebCore::Document::updateActiveStylesheets):
        
    Renamed from recalcStyleSelector.
    Invoke the new analysis functions.
        
* dom/Document.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@104060 268f45cc-cd09-0410-ab3c-d52691b4dbfc