Re-enable WebXR by default

[bsclifton]

Description

See brave/brave-core@f460ff7 for context where WebXR was disabled during the upgrade from Chromium 78 to Chromium 79

Test plan / Steps to Reproduce

1. Visit https://immersive-web.github.io/webxr-samples/
2. You should see Yes! Your browser supports WebXR and can run Virtual Reality and Augment Reality experiences if you have the appropriate hardware.
3. Visit https://immersive-web.github.io/webxr-samples/360-photos.html?usePolyfill=0

Actual result:

Status reported is Sorry, your browser does not support WebXR. and the 360 photos page doesn't work (white background)

Expected result:

You should see the Yes! Your browser supports WebXR message and the 360 photos page should work (3D image that you can pan around inside of)

Reproduces how often:

100%

Brave version (brave://version info)

1.7.13 Chromium: 80.0.3987.122 (Official Build) nightly (64-bit)

Version/Channel Information:

All channels

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

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? N
• Does the issue resolve itself when disabling Brave Rewards? N
• Is the issue reproducible on the latest version of Chrome? N

[bsclifton]

Privacy / security review: https://github.com/brave/security/issues/60

[kjozwiak]

Verification PASSED on macOS 10.15.3 x64 Catalina using the following build:

Brave | 1.7.71 Chromium: 80.0.3987.149 (Official Build) dev (64-bit)
-- | --
Revision | 5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS | macOS Version 10.15.3 (Build 19D76)

• Verified the STR/Test Plan outlined via Re-enable WebXR by default #8424 (comment)
• ensured that you can pan around https://immersive-web.github.io/webxr-samples/360-photos.html?usePolyfill=0

Verification passed on

Brave	1.7.71 Chromium: 80.0.3987.149 (Official Build) dev (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	Ubuntu 18.04 LTS

Verified test plan from the description

Verification passed on

Brave	1.7.73 Chromium: 80.0.3987.149 (Official Build) dev (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

Verified test plan from the description