Flush the SkCanvas when submitting a frame in ShellTestPlatformViewVulkan::OffscreenSurface (flutter#16717)

Author: George Wright

shell/common/shell_test_platform_view_vulkan.cc

@@ -149,8 +149,11 @@ ShellTestPlatformViewVulkan::OffScreenSurface::AcquireFrame(
                                       SkAlphaType::kOpaque_SkAlphaType);
   auto surface = SkSurface::MakeRenderTarget(context_.get(), SkBudgeted::kNo,
                                              image_info, 0, nullptr);
-  SurfaceFrame::SubmitCallback callback =
-      [](const SurfaceFrame&, SkCanvas* canvas) -> bool { return true; };
+  SurfaceFrame::SubmitCallback callback = [](const SurfaceFrame&,
+                                             SkCanvas* canvas) -> bool {
+    canvas->flush();
+    return true;
+  };
 
   return std::make_unique<SurfaceFrame>(std::move(surface), true,
                                         std::move(callback));