[SVG] State saver scrubbing.

This is a minor refactoring around GraphicsContextStateSaver usage:

* RenderSVGRoot::paintReplaced(): convert to using a state saver ->
  since everything is RAII managed now, we can simplify scoping /
  early return logic.

* PaintInfo::applyTransform(): take an optional state saver param, and
  handle the save locally if needed -> removes caller duplicated
  logic. This relies on the newly introduced
  GraphicsContextStateSaver::saveIfNeeded().

* SVGRenderingContext::prepareToRenderSVGContent():
  SaveGraphicsContext is pretty much a direct save() indicator, and
  there's no reason for its 3 users not to use an explicit state saver
  instead  -> simplifies the API & impl.

R=pdr@chromium.org,fs@opera.com,schenney@chromium.org
BUG=417783

Review URL: https://codereview.chromium.org/608563002

git-svn-id: svn://svn.chromium.org/blink/trunk@182718 bbb929c8-8fbe-4397-9dbb-9b2b20218538