This PR accomplishes many things:
- Unblocks input resampling on Fuchsia
- Fixes 2 frame scheduling issues resulting in wasted work and missed frames
- Removes the now-defunct VsyncRecorder class
- Prepares the VsyncWaiter class for the Embedder API by moving all logic out of it
- Adds hundreds of lines of unittests for Fuchsia scheduling logic to validate the performance and correctness benefits