Decouple content transform fiddling from SVGRenderingContext::renderSubtree

The "content transform" is essentially a property of the call-site, and
is hence invariant for a sequence of calls to renderSubtree.
This means we ought to be able to save-mutate-restore (SMR) once per
call-site, and not per call to renderSubtree. Achieve this by moving the
SMR to a helper SubtreeContentTransformScope, and then instantiate objects
where needed.
This means the transform-logistics can be cleaned up a bit for some of the
callers.

BUG=420022

Review URL: https://codereview.chromium.org/666403002

git-svn-id: svn://svn.chromium.org/blink/trunk@184245 bbb929c8-8fbe-4397-9dbb-9b2b20218538