Add SkBitmap::setColorSpace.

This will be useful for reimplementing PNGCodec in Chrome.

Change-Id: I63ed223a3ff39459310d2330def572b66ec02ace
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/835185
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>