Follow up #5142 SpeedReader code review

[AndriusA]

Description

Follow up for brave/brave-core#5142

Documents comments left to address.

• We're using the component installer (SpeedreaderWhitelist) where we should be using the SpeedreaderService. speedreader::Speedreader should be a base::Singleton and can be accessed inside the SpeedreaderService using Speedreader::GetInstance
• SpeedreaderWhitelist should be SpeedreaderWhitelistInstaller`
• kSpeedreaderEnabled doesn't appear to actually disable speedreader, it just removes the button (wasting cpu cycles and memory when it's off). It sort of turns it off with SpeedreaderTabHelper::UpdateActiveState, but that should use a pref listener like the button
• explore ways to use chromium's css/html parsers instead of relying on third-party library that may be treating things differently

[iefremov]

stale