Move DocumentMarker to oilpan's heap

This is a left-over of erik's CL (https://codereview.chromium.org/289323003/). It's weird to use raw pointers in heap vectors, so this CL moves DocumentMarker to the heap and replace the raw pointers with Members.

BUG=340522

Review URL: https://codereview.chromium.org/386503004

git-svn-id: svn://svn.chromium.org/blink/trunk@177905 bbb929c8-8fbe-4397-9dbb-9b2b20218538