This warning doesn't appear to be causing any failures so reverting may not be necessary. I notice that in the comments of the original CL there was concern about |feature| becoming signed in the future and so the tautological compare was left in.

We could just do something like
|static_assert(std::is_unsigned<std::remove_reference<decltype(feature)>::type>::value, "feature must be unsigned");|
in order to make sure that |feature|'s sign doesn't change without being noticed. Then we can safely get rid of the tautological compare.

sgtm, thanks for investigating mcnee@ !

@menee, thanks a lot, your investigation is correct.
How to do this?
|static_assert(std::is_unsigned<std::remove_reference<decltype(feature)>::type>::value, "feature must be unsigned");|

We finally decide to create a CL now to fix this warning.

https://chromium-review.googlesource.com/717696

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4d47a13e3d98ef30d74ae3d4671d57c097b7124e

commit 4d47a13e3d98ef30d74ae3d4671d57c097b7124e
Author: xzhan96 <xiaofeng.zhang@intel.com>
Date: Fri Oct 13 06:37:44 2017

[ServiceWorker] Fix the warning for comparison of unsigned expression

The original comparison is for the case that used_features
will become signed in the future.

BUG= 774200 

Change-Id: I9607d44bd2163f260223b23c4bddac0891a5152f
Reviewed-on: https://chromium-review.googlesource.com/717696
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508620}
[modify] https://crrev.com/4d47a13e3d98ef30d74ae3d4671d57c097b7124e/content/browser/service_worker/service_worker_provider_host.cc

Thanks.

#3: The static_assert that I suggested would let you keep using auto for the type of |feature| and then it would cause a compiler error if |feature| ever became signed, so we would know to update the code to handle signed values.
But declaring the type to be unsigned also works, since you're still comparing |feature| with |kNumberOfFeatures|.