Migrate SkSurface::flush methods to skgpu::ganesh version

Follow-up part 1 to https://skia-review.googlesource.com/c/skia/+/698237

It was not obvious to me where the correct GrDirectContext would
be for these surfaces, so I used the helper to extract the context
from the surface (if one exists) and use that.

Change-Id: I477d82659767d581cc6a7b7df36bd471796b11e9