[graphite] Fix thread race in Dawn backend

Our TSAN jobs are complaining of a race on 'fFinished' between when:
   we set it to true in the CreateRenderPipelineAsync callback
   we check it in DawnGraphicsPipeline::didAsyncCompilationFail

Change-Id: Idd6da0fde9aa096af3f10756cf57945894fd17bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1027916
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
1 file changed