Introduce SVGPaintServer

Fetch the actual fill from the various paint-server RenderSVGResources
into a new "data blob" SVGPaintServer. This makes said resources
independent of the GC, and means that GC-state can be managed more
directly by the caller.
Since this makes postApplyResource useless, it's removed, and
applyResource is renamed to preparePaintServer.
Some functions residing on RenderSVGResource is moved to SVGPaintServer.
Paint-server fetch+application is moved to updateGraphicsContext, which
is also changed to return a bool indicating if a paint-server is
available. Handling of clip-path-as-mask is also moved here.

BUG=420022

Review URL: https://codereview.chromium.org/638933002

git-svn-id: svn://svn.chromium.org/blink/trunk@183619 bbb929c8-8fbe-4397-9dbb-9b2b20218538