[castable-video] new default Chrome flag breaks discovery

[luwes]

there is a new Chrome flag that breaks the way the cast button is shown / hidden.

Delay media sink discovery until explicit user interaction with cast

The default setting requires a user to first interact with the Cast UI. Just loading the cast sdk doesn't seem to work.

Disabling this flag makes it work like before, detecting correctly there are cast devices and makes us show the cast button in our custom UI's.