Avoid using SVGElementInstance in calcViewport

Avoid using SVGElementInstance in calcViewport as it is scheduled to be removed. Instead implement the width/height transfer rules in SVGUseElement. This speeds up the SierpinskiCarpet test a lot as profiling showed the instance traversal in calcViewport was very high.

BUG=313438

Review URL: https://codereview.chromium.org/252903004

git-svn-id: svn://svn.chromium.org/blink/trunk@173258 bbb929c8-8fbe-4397-9dbb-9b2b20218538