Change DocumentMarkerController::MarkerList to a HeapVector<Member<RenderedDocumentMarker>>

Before this CL, MarkerList used HeapVector<RenderedDocumentMarker> and other code used HeapVector<Member<DocumentMarker>> to point to the addresses of the RenderedDocumentMarkers in the MarkerList. This is dangerous because the RenderedDocumentMarkers in the MarkerList do not have object headers. (See https://codereview.chromium.org/386503004/ for more details.)

The most straightforward fix is to make the RenderedDocumentMarkers in the MarkerList heap-allocated. This CL makes the change.

BUG=340522

Review URL: https://codereview.chromium.org/384343002

git-svn-id: svn://svn.chromium.org/blink/trunk@178228 bbb929c8-8fbe-4397-9dbb-9b2b20218538