+vivek who did the previous move _to_ Blink-in-JS and may have context
+haraken,esprehn for their opinions

SGTM. This looks like it would more or less amount to a revert of the changes that moved it to Blink-in-JS (except for any bug fixes there may have been in the meantime).

Those changes were (in multiple pieces, because we had split repositories at the time):
https://codereview.chromium.org/479753003
https://codereview.chromium.org/539653002
https://codereview.chromium.org/540493002

It seems we had originally used the main world for this (so presumably that's web-compatible), but an isolated world shouldn't have a high cost here and does insulate the script more from other frames.

Go ahead!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e2ebe3280b99674d33fc8f3c4c006080168afa51

commit e2ebe3280b99674d33fc8f3c4c006080168afa51
Author: adithyas <adithyas@chromium.org>
Date: Tue Dec 13 18:11:01 2016

Move DocumentXMLTreeViewer off of private script

- Execute DocumentXMLTreeViewer.js in an isolated world
- Remove all private script hooks
- Delete idl file

BUG= 672500 

Review-Url: https://codereview.chromium.org/2562093002
Cr-Commit-Position: refs/heads/master@{#438224}

[modify] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/content/child/blink_platform_impl.cc
[modify] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/Source/bindings/core/v8/DOMWrapperWorld.h
[modify] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/Source/core/BUILD.gn
[modify] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/Source/core/core_idl_files.gni
[modify] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/Source/core/xml/BUILD.gn
[add] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/Source/core/xml/DocumentXMLTreeViewer.cpp
[add] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/Source/core/xml/DocumentXMLTreeViewer.h
[delete] https://crrev.com/2a1f622ddc372a0bd4206fb01c45f4ebbe37258b/third_party/WebKit/Source/core/xml/DocumentXMLTreeViewer.idl
[modify] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/Source/core/xml/DocumentXMLTreeViewer.js
[modify] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/Source/core/xml/parser/XMLDocumentParser.cpp
[modify] https://crrev.com/e2ebe3280b99674d33fc8f3c4c006080168afa51/third_party/WebKit/public/blink_resources.grd