Oilpan: |SVGViewSpec::m_contextElement| should be a member.

r175772 changed lifetime of SVGViewSpec to be always same as SVGSVGElement,
so this weak-ref can now be made a strong-ref.

BUG=357163

Review URL: https://codereview.chromium.org/331583002

git-svn-id: svn://svn.chromium.org/blink/trunk@176001 bbb929c8-8fbe-4397-9dbb-9b2b20218538