background takes too long to turn dark when opening downloaded media from WebTorrent

[kjozwiak]

Description

As per #5326 (comment), the background shouldn't be taking this long to switch from Light -> Dark. It also shouldn't cover/hide content/images that are dark that might blend into the background.

Steps to Reproduce

1. Visit this URL: https://ia800301.us.archive.org/14/items/art_of_war_librivox/art_of_war_librivox_archive.torrent
2. Start the torrent download
3. Click on file 12 "art_of_war_01-02_sun_tzu.png" and confirm that the image shows up.

You'll notice that the entire background will go dark after a short delay which also covers up the image.

Actual result:

Expected result:

Background shouldn't take this long to turn dark and shouldn't be covering the media that's opened.

Reproduces how often:

100% reproducible using the above STR.

Brave version (brave://version info)

Brave	0.69.128 Chromium: 77.0.3865.75 (Official Build) (64-bit)
Revision	201e747d032611c5f2785cae06e894cf85be7f8a-refs/branch-heads/3865@{#776}
OS	macOS Version 10.14.6 (Build 18G95)

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 dev channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

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

Miscellaneous Information:

CCing @feross

[feross]

Thanks for the issue report.