Skip flush in GrDrawingManager if the specifed proxy doesn't have any work

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

Change-Id: Id359ad5a01a290e7e6c06f7ccc1c385ad47d2c06
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206277
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed