[graphite] Fix synchronous readback on WebGPU

Bug: b/336779772
Change-Id: I7aebd99e99b543d3e3ae87c02abd92f6a1af7d83
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/857336
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
1 file changed