Reviewed by Oliver Hunt.
        
        Added HashMap::swap and HashSet::swap. WebCore now uses HashSet::swap.
        I figured while I was in the neighborhood I might as well add HashMap::swap,
        too.

        * wtf/HashMap.h:
        (WTF::::operator):
        (WTF::::swap):
        * wtf/HashSet.h:
        (WTF::::operator):
        (WTF::::swap):

WebCore:

        Reviewed by Oliver Hunt.
        
        Some improvements on my last patch, suggested by Darin.

        * history/PageCache.cpp:
        (WebCore::PageCache::autoreleaseNow): Swap with an empty set instead of 
        building up an array and then calling clear(). This is slightly more 
        efficient and the code is cleaner.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@21468 268f45cc-cd09-0410-ab3c-d52691b4dbfc