DevTools heap snapshot currently captures DOM nodes but does not show their sizes.

In order to show sizes we need to:
1) plumb size info from the Oilpan GC to the EmbedderGraphBuilder in Blink.
2) adjust DevTools UI to show both JS heap size and DOM size separately to avoid confusion when a JS developer compares performance.memory with the heap snapshot. (Currently DevTools shows only the JS heap size as the heap snapshot size).