Reduce usage of SkCanvas::flush

SkCanvas::flush is problematic and we wish to deprecate it. As a first step, this CL begins to remove Skia's internal usage of it.

Ideally clients would use SkSurface::flush and/or GrContext::flush.

Change-Id: I39bb0702f8230134a97961a4ee70833fd5bd0dcc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/196641
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
31 files changed