Added a thrash check to live decoded resource eviction.
        
        Here's the strategy: Stamp every image with its paint time. Don't evict 
        a live decoded resource until another resource paints with a reasonably
        (1 second) larger time stamp. 
        
        If no other resource paints, or another resource paints, but very soon 
        after the resource in question, the resource in question is very likely 
        to paint again soon. In fact, it's probably still on screen. So we 
        leave it alone. (Previously, we evicted it on a timer, but that would 
        evict a resource that was still on screen, hurting speed without 
        helping memory use.)

        In theory, this algorithm allows a single large resource or closely 
        related set of resources to linger in the live decoded cache even 
        though the cache is over its limit. However, that can only happen as 
        long as no other resource ever paints again, which guarantees an 
        absolute cap on cache memory usage from then on. Also, the resources 
        will only linger as long as they remain live. Upon going dead, they 
        will flush. Also, these circumstances are so rare that they are almost 
        impossible to encounter in the wild. So don't sweat it.
        
        Stop evicting if the next resource painted too recently:
        
        * loader/Cache.cpp:
        (WebCore::Cache::pruneLiveResources):
        * loader/CachedResource.cpp:
        (WebCore::CachedResource::CachedResource):
        (WebCore::CachedResource::liveResourceAccessed):
        * loader/CachedResource.h:

        Track the paint time stamp in Frame. We do this to give a consistent
        stamp to all resources painted in a single paint operation (in case the
        operation takes a significant amount of time), and to avoid excessive 
        calls to system time functions, which hurt the PLT:

        * page/Frame.cpp:
        (WebCore::Frame::paint):
        * page/Frame.h:
        (WebCore::Frame::currentPaintTimeStamp):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@24951 268f45cc-cd09-0410-ab3c-d52691b4dbfc