Move SVGRareData bit field from Node to SVGElement

This frees up a bit in Node, and since SVGElement already had a bunch
of bools we can switch them to a bitfield and not grow the memory usage
there either.

Review URL: https://codereview.chromium.org/118373005

git-svn-id: svn://svn.chromium.org/blink/trunk@164127 bbb929c8-8fbe-4397-9dbb-9b2b20218538