Backward trickPlay() works while paused, but forward trickPlay() does not

[joeyparrish]

Have you read the FAQ and checked for duplicate open issues? Yes

What version of Shaka Player are you using? master

Can you reproduce the issue with our latest release version? Yes

Can you reproduce the issue with the latest code from master? Yes

Are you using the demo app or your own custom app? Demo

What browser and OS are you using? ChromeOS 71, but it should be common to all platforms

What are the manifest and license server URIs? Any

What did you do?

1. Load a video
2. Seek to the middle
3. Pause the video
4. Use player.trickPlay() API or equivalent UI elements to rewind and fast-forward

What did you expect to happen?
I expected fast-forward and rewind to work while paused.

What actually happened?
Rewind (negative trickPlay() rate) worked while paused, because it is implemented as a series of seeks. Fast forward (positive trickPlay() rate) did not work, because it is implemented with playbackRate. Rewind is not implemented with playbackRate because negative values are not supported.