I dont think this code adds much value, as SVGLocatable::getBBox is not used much out there.
Created attachment 141450 [details]
I ran run-perf-tests on the svg PageLoad performance tests and there were no speed regressions there (makes sense since the changed code path is not called by these heavy svg tests). From our layout tests only svg/custom/getBBox-path.svg uses it, and there it is only asked once, so caching is not useful for this test. So overall I think the benefits outweigh the drawbacks of removing this caching, I just don't see this as a hot code path.
Comment on attachment 141450 [details]
Good idea, this was more beneficial in the past where renderers grabbed bounds using getBBox().
Committed r117696: <http://trac.webkit.org/changeset/117696>