Mark SVGElementRareData::m_owner as oilpan-only

Mark SVGElementRareData::m_owner as oilpan-only, it takes extra space for non-oilpan builds.

BUG=

Review URL: https://codereview.chromium.org/296093002

git-svn-id: svn://svn.chromium.org/blink/trunk@174485 bbb929c8-8fbe-4397-9dbb-9b2b20218538