URLDownloader and the corresponding unit tests have the following problem:

- URLDownloader uses private inheritance from reading_list::ReadingListDistillerPageDelegate (forbidden, all inheritance should be public according to https://google.github.io/styleguide/cppguide.html#Inheritance)

- URLDownloader unit tests does not use the public API, but instead define a sub-class of URLDownloader that overrides random methods and is friend with the super class (this breaks the rule of unit tests that should only test the public API)

- URLDownloader unit tests use busy loop with timeout to wait for internal state of the sub-class of URLDownloader to change to detect that some condition happened (this introduce race-condition and this reveal that the public API of URLDownloader does not allow client to use it reliably).

The later point causes issue with TaskScheduler migration (as the race-condition is exhibited reliably causing the test to become flaky).