Reland "DevTools: Add ARIAUtils.setCheckboxAsIndeterminate"

This reverts commit cfc19ddd.

Reason for revert: Initial revert didn't fix failures.

Original change's description:
> Revert "DevTools: Add ARIAUtils.setCheckboxAsIndeterminate"
> 
> This reverts commit 2cf002af.
> 
> Reason for revert: Breaks http/tests/devtools/a11y-axe-core/basic-a11y-test.js and 5 other(s), see issue 1004150 - locally identified
> tests as fixed when reverting this CL (on Linux)
> 
> Original change's description:
> > DevTools: Add ARIAUtils.setCheckboxAsIndeterminate
> >
> > This adds a new API to UI.ARIAUtils to set an element's aria-checked
> > attribute to 'mixed', which is the equivalent of setting
> > intedeterminate=true on an actual checkbox element.
> > This is needed to support keyboard accessibility in the javascript and
> > event listener breakpoints views.
> >
> > Equivalent to the Animation checkbox in this screenshot:
> > https://gyazo.com/99f2067f6810af3366943176393d8b2f
> >
> > Bug: 963183
> > Change-Id: Ie09f8e993d1776774c410f6099356f3a1d912048
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715815
> > Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
> > Auto-Submit: Jack Lynch <jalyn@microsoft.com>
> > Commit-Queue: Jack Lynch <jalyn@microsoft.com>
> > Cr-Commit-Position: refs/heads/master@{#696551}
> 
> TBR=pfeldman@chromium.org,einbinder@chromium.org,jalyn@microsoft.com
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Bug: 963183, 1004150
> Tbr: ortuno@
> Change-Id: I3ef2a4a5b74be00de669db215f27c2586acba40f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803937
> Commit-Queue: Dominik Röttsches <drott@chromium.org>
> Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
> Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
> Reviewed-by: Dominik Röttsches <drott@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#696700}

TBR=pfeldman@chromium.org,drott@chromium.org,ortuno@chromium.org,einbinder@chromium.org,jalyn@microsoft.com

Change-Id: I433a1549e7f1ef909fe2b3b0fc2cea584bceafcd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 963183, 1004150
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804703Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696708}

third_party/blink/renderer/devtools/front_end/ui/ARIAUtils.js

@@ -236,6 +236,13 @@ UI.ARIAUtils.setChecked = function(element, value) {
   element.setAttribute('aria-checked', !!value);
 };
 
+/**
+ * @param {!Element} element
+ */
+UI.ARIAUtils.setCheckboxAsIndeterminate = function(element) {
+  element.setAttribute('aria-checked', 'mixed');
+};
+
 /**
  * @param {!Element} element
  * @param {boolean} value