Oilpan: Make DateTimeEditElement::m_editControlOwner and DateTimeFieldElement::m_fieldOwner strong references.

We don't need to use WeakMember for them because owners never die before owned
elements.
In non-Oilpan, owners can die before owned elements because parentNode is not a
strong reference.

BUG=357163

Review URL: https://codereview.chromium.org/275513003

git-svn-id: svn://svn.chromium.org/blink/trunk@173590 bbb929c8-8fbe-4397-9dbb-9b2b20218538