Taking a look.

Patch up at https://codereview.chromium.org/2306923002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fade7ae22a3ea448947c5c90cb1a2ac2b5c30781

commit fade7ae22a3ea448947c5c90cb1a2ac2b5c30781
Author: dominicc <dominicc@chromium.org>
Date: Sun Sep 04 01:00:58 2016

Prevent recursion in critical part of CustomElementRegistry::define.

Custom element registry used to allow recursion, but this complicated
the check for whether a name was in use. Now define [1] has an simpler
explicit recursion guard called the "element definition is running
flag." [2]

[1] https://html.spec.whatwg.org/#dom-customelementregistry-define
[2] https://html.spec.whatwg.org/#element-definition-is-running

BUG= 643051 

Review-Url: https://codereview.chromium.org/2306923002
Cr-Commit-Position: refs/heads/master@{#416460}

[modify] https://crrev.com/fade7ae22a3ea448947c5c90cb1a2ac2b5c30781/third_party/WebKit/LayoutTests/custom-elements/spec/define-element.html
[modify] https://crrev.com/fade7ae22a3ea448947c5c90cb1a2ac2b5c30781/third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.cpp
[modify] https://crrev.com/fade7ae22a3ea448947c5c90cb1a2ac2b5c30781/third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.h

 Issue 640850  has been merged into this issue.