Fixing WindowManagerTest::DialogCanNeverBeFullscreen possibly hanging (flutter#177179)

fixes flutter#177172

I believe the issue is that the struct was not fully initialized (which
is usually not a problem, but can sometimes get garbage values and start
hanging). The solution is probably to initialize the struct. Let's see
if it fixes CI :)

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- [x] I listed at least one issue that this PR fixes in the description
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- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
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Author: mattkae

engine/src/flutter/shell/platform/windows/window_manager_unittests.cc

@@ -368,13 +368,12 @@ TEST_F(WindowManagerTest, DialogCanNeverBeFullscreen) {
   IsolateScope isolate_scope(isolate());
 
   DialogWindowCreationRequest creation_request{
-      .preferred_size =
-          {
-              .has_preferred_view_size = true,
-              .preferred_view_width = 800,
-              .preferred_view_height = 600,
-          },
-  };
+      .preferred_size = {.has_preferred_view_size = true,
+                         .preferred_view_width = 800,
+                         .preferred_view_height = 600},
+      .preferred_constraints = {.has_view_constraints = false},
+      .title = L"Hello World",
+      .parent_or_null = nullptr};
 
   const int64_t view_id =
       InternalFlutterWindows_WindowManager_CreateDialogWindow(