Bug 1361274. There's no need to update link :visited state when doing querySelectorAll, since querySelectorAll ignores that state anyway. r=smaug

In our current setup, in which links with an href attribute always match either
:link or :visited, no matter whether that attribute's value is a valid URI,
changes to the attribute always put the element into either the "match nothing"
state or the "match :link" state, via calls to Link::ResetLinkState.

The only thing FlushPendingLinkUpdates is needed for is (lazily, in case it
turns out to not be needed because the element got removed from the DOM anyway)
registering a history observer to switch the link state to :visited as needed.

This means that selector matching consumers that would never expose :visited
state to start with don't need to worry about calling FlushPendingLinkUpdates.

dom/html/HTMLContentElement.cpp

@@ -295,7 +295,6 @@ HTMLContentElement::Match(nsIContent* aContent)
 
     TreeMatchContext matchingContext(false, nsRuleWalker::eRelevantLinkUnvisited,
                                      doc, TreeMatchContext::eNeverMatchVisited);
-    doc->FlushPendingLinkUpdates();
     matchingContext.SetHasSpecifiedScope();
     matchingContext.AddScopeElement(host->AsElement());