Good catch! This seems to only affect DCHECK_IS_ON code and has never worked correctly since originally added as a WTF::TemporaryChange over two years ago:

https://chromium.googlesource.com/chromium/src/+/a973a859a9b6c995ff5081174426306a9fa3914d

Given that, leaving it at P3 and letting mlippautz@ decide whether we want to correct this scoped AutoReset or eliminate the flag in question.

Nice find :)

The scope is used to avoid adding unnecessary objects to the verifier. For the time being we'd still like to have the verifier.  Lets fix the scope by giving it a name. This will speed up debug build by a bit although I am not sure if it's measurable these days.

Can do that on Monday unless you want to fix it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/38e1c72978cd85dae716ddab01f8ad6e66cb6b61

commit 38e1c72978cd85dae716ddab01f8ad6e66cb6b61
Author: Michael Lippautz <mlippautz@chromium.org>
Date: Mon Sep 17 10:02:33 2018

[wrapper-tracing] Remove tracing scope in AdvanceTracing

The wrapper tracing verifier used to only record entry points in the Blink
graph as the verification pass then transitively traversed the graph again to
check for non-black objects. To do so, only objects that have an incoming V8
reference were recorded for verification.  The broken scope was supposed to
implement that filter.

In these days however, the verifier only checks the next level of an object and
does not recurse. This requires that all objects are recorded for verification.
Remove the non-working unnecessary scope as a consequence.

Bug:  chromium:884218 
Change-Id: If37bc1815f346a5c9ae2588972761c177bc10880
Reviewed-on: https://chromium-review.googlesource.com/1227798
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591631}
[modify] https://crrev.com/38e1c72978cd85dae716ddab01f8ad6e66cb6b61/third_party/blink/renderer/platform/bindings/script_wrappable_marking_visitor.cc
[modify] https://crrev.com/38e1c72978cd85dae716ddab01f8ad6e66cb6b61/third_party/blink/renderer/platform/bindings/script_wrappable_marking_visitor.h