[SVG] Make SVGViewSpec use a WeakPtr to hold reference to contextElement.

m_contextElement was previously a raw ptr, manually invalidated from ~SVGSVGElement.

This patch changes it to use WeakPtr so this behavior is explicit.

Review URL: https://codereview.chromium.org/77493002

git-svn-id: svn://svn.chromium.org/blink/trunk@162339 bbb929c8-8fbe-4397-9dbb-9b2b20218538