We might want to suppress the warning only for chrome:// pages.
Is this unfair??

Re: comment 1, I don't think we should do this. We need to work with the Web UI team instead to remove the usage of /deep/.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/afe8b19377eba0d66ce30c8753feee2e92fc767b

commit afe8b19377eba0d66ce30c8753feee2e92fc767b
Author: kochi <kochi@chromium.org>
Date: Tue Mar 15 00:50:25 2016

Do not show deprecation message for ::shadow and /deep/ on querySelector

For ::shadow pseudo element and /deep/ combinator, which is deprecated
for CSS but still is allowed for querySelector[All].

BUG= 594458 

Review URL: https://codereview.chromium.org/1796573005

Cr-Commit-Position: refs/heads/master@{#381140}

[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/LayoutTests/fast/dom/SelectorAPI/only-shadow-host-in-shadow-tree-expected.txt
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/LayoutTests/fast/dom/shadow/all-in-shadow-tree-expected.txt
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/LayoutTests/fast/dom/shadow/querySelector-for-multiple-shadowroots-expected.txt
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/LayoutTests/fast/dom/shadow/querySelector-for-useragent-shadowroot-expected.txt
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/LayoutTests/fast/dom/shadow/querySelector-with-detached-node-distribution-expected.txt
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/LayoutTests/fast/dom/shadow/querySelector-with-distribution-expected.txt
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/LayoutTests/fast/dom/shadow/querySelector-with-shadow-all-and-shadow-deep-expected.txt
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/LayoutTests/shadow-dom/closed-mode-deep-combinators-expected.txt
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/Source/core/css/SelectorChecker.h
[modify] https://crrev.com/afe8b19377eba0d66ce30c8753feee2e92fc767b/third_party/WebKit/Source/core/dom/SelectorQuery.cpp

Regarding comment 1, do you think it is worth allowing /deep/ & ::shadow
in CSS selector on chrome:// pages?

As Blink is not the engine only for Chromium/Chrome, I don't think handling
chrome:// pages specially in Blink cannot be accepted.