Update SVGResourcesCache on SVG root insertion and removal

SVG renderers update the SVGResourcesCache when children are added or
removed using the addChild and removeChild methods. The root SVG
renderer (RenderSVGRoot) may not properly update its own resources
cache because its parent will not go through the SVG
addChild/removeChild functions.

This patch updates RenderSVGRoot to hook insertedIntoTree and
willBeRemovedFromTree to update the resources cache for the root.

BUG=277848

Review URL: https://chromiumcodereview.appspot.com/23002011

git-svn-id: svn://svn.chromium.org/blink/trunk@156672 bbb929c8-8fbe-4397-9dbb-9b2b20218538