Refactor DocumentMarker
Eliminate DocumentMarkerDetails in favor of making DocumentMarker a polymorphic
class hierarchy (since we're already subclassing it for TextMarker anyway)
Depends on
https://codereview.chromium.org/2763893002/
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
BUG=
707867