DevTools [Network Conditions]: making network throttling options read optgroup label

Problem:
- NVDA does not currently support reading <optgroup> elements in Chromium
- Optgroup labels are inaccessible in Network throttling select element in Network Conditions tab

Changes:
- Select options are given a new accessible name that prefaces the option title with the optgroup title.

Bug: 963183
Change-Id: Id9a71997c96d3646c7cf35de94213bccf5e68912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730863Reviewed-by: John Emau <johnemau@microsoft.com>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Michael Liao <michael.liao@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#701660}

third_party/blink/renderer/devtools/front_end/mobile_throttling/ThrottlingManager.js

@@ -54,11 +54,14 @@ MobileThrottling.ThrottlingManager = class extends Common.Object {
         for (const conditions of group.items) {
           const title = conditions.title;
           const option = new Option(title, title);
+          UI.ARIAUtils.setAccessibleName(option, ls`${group.title}: ${title}`);
           groupElement.appendChild(option);
           options.push(conditions);
         }
         if (i === groups.length - 1) {
-          groupElement.appendChild(new Option(Common.UIString('Add\u2026'), Common.UIString('Add\u2026')));
+          const option = new Option(ls`Add\u2026`, ls`Add\u2026`);
+          UI.ARIAUtils.setAccessibleName(option, ls`Add ${group.title}`);
+          groupElement.appendChild(option);
           options.push(null);
         }
       }

third_party/blink/renderer/devtools/front_end/mobile_throttling/mobile_throttling_strings.grdp

@@ -63,6 +63,9 @@
   <message name="IDS_DEVTOOLS_b27fdc01c81857f96f3d5d37d323bcb6" desc="Title of an action in the network conditions tool to network mid tier mobile">
     Enable fast 3G throttling
   </message>
+  <message name="IDS_DEVTOOLS_b95c09fb28e80cc672fd533a62252d70" desc="Accessibility label for custom add network throttling option">
+    Add <ph name="GROUP_TITLE">$1s<ex>Custom</ex></ph>
+  </message>
   <message name="IDS_DEVTOOLS_c47c85f963782364d32f428fcf7631ff" desc="Text in Throttling Presets of the Network panel">
     No internet connectivity
   </message>