Add samples to Viewer.

This adds support with animation, assuming the sample has
implemented onAnimate. Event handling has not been
implemented.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2056343004

Committed: https://skia.googlesource.com/skia/+/76963e73704a42a18c29d6fbdcccb566e5c67658
Review-Url: https://codereview.chromium.org/2056343004
18 files changed