Check for work completion before purging resources.

Change-Id: I17653e036ceb5f505284ae64cf6918b3bd2e02ce
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310762
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>

src/gpu/GrContext.cpp

@@ -212,6 +212,7 @@ void GrContext::performDeferredCleanup(std::chrono::milliseconds msNotUsed) {
         return;
     }
 
+    this->checkAsyncWorkCompletion();
     fMappedBufferManager->process();
     auto purgeTime = GrStdSteadyClock::now() - msNotUsed;