Add tracing to ResourceLoadPriorityOptimizer::updateAllImageResourcePriorities

I discovered while looking into LayoutState's fast
coordinate mapping that we can potentially do a lot
of localToAbsolute calls to determine if images are
onscreen in ResourceLoadPriorityOptimizer. These
calls are out-of-band from the RenderTree walk
done during layout, so don't benefit from the fast-
lookups. That made me believe the function may
actually be expensive.

Review URL: https://codereview.chromium.org/342763002

git-svn-id: svn://svn.chromium.org/blink/trunk@176455 bbb929c8-8fbe-4397-9dbb-9b2b20218538