Tab and spacebar keyboard-accessibility don't work on Widevine panel

[stephendonner]

Description

Tab and spacebar keyboard-accessibility don't work on the Widevine panel. This might be a more-general/widespread problem - I did search for duplicate bugs, though.

Steps to Reproduce

1. new profile, launch Brave
2. load https://bitmovin.com/demos/drm
3. when the Widevine popup appears, try pressing tab to forward-cycle through the browser's UI elements
4. click the "Don't ask again" checkbox
5. now press the spacebar
6. now press tab again

Actual results:

3. tabing doesn't do anything
4. there's no focus ring around the checkbox and its text
5. nothing happens
6. nothing happens

Expected results:

3. pressing tab should forward-cycle (using tab-index) through UI elements, focusing them with each press
4. there should be a focus ring around the checkbox and its text
5. pressing the spacebar should toggle the checked/unchecked state of the "Don't ask again" checkbox
6. tabbing from the Don't ask again checkbox should logically focus the Block button element, I'd think

Reproduces how often:

Brave version (brave://version info)

Brave	1.23.23 Chromium: 89.0.4389.72 (Official Build) nightly (x86_64)
Revision	3f345f156bfd157bd1bea06310e55f3fb2490359-refs/branch-heads/4389@{#1393}
OS	macOS Version 11.2.3 (Build 20D91)

Version/Channel Information:

• Can you reproduce this issue with the current release? yes
• Can you reproduce this issue with the beta channel? yes
• Can you reproduce this issue with the nightly channel? yes

cc: @karenkliu