DevTools: Make Settings Blackboxing Pane Accessible
DevTools: Make Settings Blackboxing Pane Accessible

This change provides fixes for these issues:
Problem: The content title was heading role
Fix: Added heading role of level 1
Problem: Aria-label was missing in drop-downs
Fix: Added aria-label in drop-downs

Tested keyboard navigation
- navigate up/down drop-down items using arrow keys
- Checkbox is checked/unchecked using space bar
- Tab into/out (no tab traps)

Tested screen reader
- reads checkbox label
- reads items selected from the drop down list

Bug: 963183

Change-Id: Iaab2dd37323427215f765b3d7bbef885861c210f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1605506
Commit-Queue: Chandani Shrestha <52538cdd559ab5325fe6b80437501fc583a9e806@microsoft.com>
Reviewed-by: Joel Einbinder <bab755abe9fee48fe544272e55f73517d4e4a725@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#660991}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b8296ec2e06868d1365a61bbf0f283e297682252

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