Add doc comments to WindowsTestContext (#35350)

Adds missing doc comments to WindowsTestContext.

Issue: https://github.com/flutter/flutter/issues/87299
diff --git a/shell/platform/windows/testing/windows_test_context.h b/shell/platform/windows/testing/windows_test_context.h
index bdc6fcc..30673f1 100644
--- a/shell/platform/windows/testing/windows_test_context.h
+++ b/shell/platform/windows/testing/windows_test_context.h
@@ -23,8 +23,10 @@
   explicit WindowsTestContext(std::string_view assets_path = "");
   virtual ~WindowsTestContext();
 
+  // Returns the path to assets required by the Flutter runtime.
   const std::wstring& GetAssetsPath() const;
 
+  // Returns the path to the ICU library data file.
   const std::wstring& GetIcuDataPath() const;
 
  private: