SVGViewSpec wrapper should keep its context SVGSVGElement wrapper alive.

There seems to be some cases where a SVGSVGElement wrapper is gc-ed while the SVGSVGElement is alive.
However, |m_transform| and other properties held by SVGViewSpec manage its lifetime via the SVGSVGElement v8 wrapper.

This patch avoids the situation by keeping the context SVGSVGElement wrapper alive from SVGViewSpec wrapper.

BUG=379998

Review URL: https://codereview.chromium.org/315893002

git-svn-id: svn://svn.chromium.org/blink/trunk@175772 bbb929c8-8fbe-4397-9dbb-9b2b20218538