Oilpan: Change LoadableTextTrack::m_trackElement a strong Member

This CL changes LoadableTextTrack::m_trackElement from a WeakMember to a Member.

In non-oilpan, LoadableTextTrack::m_trackElement is implemented as a weak pointer,
but it's just because there is no way in non-oilpan to make it a strong pointer
without creating a cycle. Ideally LoadableTextTrack::m_trackElement should be
implemented as a strong pointer. This is possible in oilpan, and this CL makes the change.

BUG=340522

Review URL: https://codereview.chromium.org/351563005

git-svn-id: svn://svn.chromium.org/blink/trunk@177052 bbb929c8-8fbe-4397-9dbb-9b2b20218538