Add missing flush in SkiaOutputSurfaceImplTest.SupportsColorSpaceChange

A missing flush in SkiaOutputSurfaceImplTest.SupportsColorSpaceChange
was causing the test to fail when run with SwANGLE. Adding it fixes the
issue.

Bug: angleproject:5338
Change-Id: I3c098e66af738535e3da495192e5c37a3d34fba5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537998Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827326}

components/viz/service/display_embedder/skia_output_surface_impl_unittest.cc

@@ -192,6 +192,7 @@ TEST_F(SkiaOutputSurfaceImplTest, SupportsColorSpaceChange) {
     OutputSurfaceFrame frame;
     frame.size = kSurfaceRect.size();
     output_surface_->SwapBuffers(std::move(frame));
+    output_surface_->Flush();
 
     run_loop.Run();
   }