Remove SVGElementInstance

The SVGElementInstance tree has been removed from SVG2:

http://www.w3.org/TR/SVG2/changes.html#structure

This patch removes it completely. The exact naming of the various trees after removing the instance tree is still under discussion and will be implemented in a follow-up patch.

BUG=313438

Review URL: https://codereview.chromium.org/272523002

git-svn-id: svn://svn.chromium.org/blink/trunk@175681 bbb929c8-8fbe-4397-9dbb-9b2b20218538