Make DocumentMarker::GetType() virtual

Now that we have subclasses of DocumentMarker for all the different MarkerTypes,
we can make GetType() a virtual method on each of the subtypes so we don't
actually have to store a field in DocumentMarker for this information.

BUG=707867

Review-Url: https://codereview.chromium.org/2911733002
Cr-Commit-Position: refs/heads/master@{#476473}
13 files changed