Oilpan: Remove Element* raw pointers in objects

- Make StyleResolver's StyleSharingList a HeapDeque of Member<Element>s.
- Make CSSFontFaceSrcValue::m_svgFontFaceElement a Member<Element>.
- Add a FIXME to make ScriptLoader::m_element to a Member<Element>.
Given that ResourceClient is going to be moved to the heap soon,
it would be better to just wait for it rather than making it a Persistent<Element>.

BUG=340522

Review URL: https://codereview.chromium.org/423013002

git-svn-id: svn://svn.chromium.org/blink/trunk@179086 bbb929c8-8fbe-4397-9dbb-9b2b20218538