[worklets] Add language for non-determinism in a note.

[bfgeek]

This adds a note that user-agents must introduce some non-determinism into the selection process for a paint worklet global scope.

This came up in blinks intent-to-ship thread: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/Jex3idOld48

Issue: #471

[foolip]

Some thoughts on how to test this. Could the spec have a limit to say that the same scope cannot be used more than n times in a row, where n is a large number? Then the implementation could be to randomly pick a scope and randomly switch, but a test that checks that it does eventually switch would both be supported by what the spec says, and would pass in a reasonable time frame.