* Tweak background video playback description in flag page
* Remove redundant call to allow Javascript injection
The method `content::RenderFrameHost::AllowInjectingJavaScript()` is already called in
`background_video_playback_tab_helper` for those domains that correctly have a match so
it's not needed in `brave_browser_main_parts`.
* Fix boolean logic
* Make `IsBackgroundVideoPlaybackEnabled` method more readable
* Remove unneeded predefined default value from XML
* Refactor logic and remove old code
* Enable background video playback preference by default
* Remove unused constant
Simone Arpe
660020c778