Accommodate pre-existing __onGCastApiAvailable

[jkoppen-bluebillywig]

Have you read the FAQ and checked for duplicate open issues?
Yup. No open duplicates. Next of kin: #3167

Is your feature request related to a problem? Please describe.
In our project we use -- and consequently load -- Google Cast framework. Hence, we register a callback in window.__onGCastApiAvailable . Then, when we initialize shaka.cast.CastProxy it gives up, throwing error 8000 ('CAST_API_UNAVAILABLE').

Describe the solution you'd like
The shaka.CastProxy could save any pre-existing window.__onGCastApiAvailable before registering its own callback. Then, when the framework becomes available, it could execute its own business logic, and restore __onGCastApiAvailable and call it (with the appropriate argument).

Describe alternatives you've considered
The only alternative I see is initializing shaka.cast.CastProxy before loading Google Cast framework, as our code already does play nicely with pre-existing window.__onGCastApiAvailable . That, however, would imply a restructuring from the ground up. Currently a specific "media endpoint" (e.g. Shaka endpoint) is only summoned into existence when needed for playing a specific media format (e.g. MPEG-Dash).

Additional context
A PR of the proposed solution will be provided.

[jkoppen-bluebillywig]

#4523