Reference counting differences between Chromium and DRT causes crash in Chromium that can't be reproduced in DRT.
In this case it's because accessibility helper objects indirectly hold extra reference to Frame objects. Perhaps conditionally enable/disable them to make behaviour more similar.
Unimplemented DRT features are being marked WontFix. If they cause tests to fail, we'll see it. If not, we're switching to ContentShell and don't want to put effort into DRT.