Add callback to flush for knowing when gpu is finished work.

Bug: skia:8802
Change-Id: I093c2a4e879b635b169a849d9af3e9f7a3d84a88
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207870
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 6bce617..9436d7c 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -95,6 +95,7 @@
   "$_tests/GrCCPRTest.cpp",
   "$_tests/GrContextAbandonTest.cpp",
   "$_tests/GrContextFactoryTest.cpp",
+  "$_tests/GrFinishedFlushTest.cpp",
   "$_tests/GrGLExtensionsTest.cpp",
   "$_tests/GrMemoryPoolTest.cpp",
   "$_tests/GrMeshTest.cpp",