Skip flush in GrDrawingManager if the specifed proxy doesn't have any work (take 2)

This is rather ham-fisted but I would like to have a short-term fix for the "always-flush" perf regressions.

Change-Id: I508e2d725ac18f50318baf31bc3243fe932c724b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206697
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
18 files changed