Improve AnimationWorklet README.

- Introduce 3 different contexts that matter: main, compositor, animation (with a picture!)
- Introduce concept of a worklet animation and its incarnations in each context
- Explain some details about what is involves in an animation frame
- Short discussion of compositor and its current basic scheduling.


Bug: 430155
Change-Id: Ida4da0762d6b212d080eb01f25986e179ab81828
Reviewed-on: https://chromium-review.googlesource.com/825247
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529338}
2 files changed