[windows] Add horizontal scroll support

shell/platform/windows/BUILD.gn

@@ -123,10 +123,10 @@ executable("flutter_windows_unittests") {
   sources = [
     "string_conversion_unittests.cc",
     "system_utils_unittests.cc",
+    "testing/mock_win32_window.cc",
+    "testing/mock_win32_window.h",
     "testing/win32_flutter_window_test.cc",
     "testing/win32_flutter_window_test.h",
-    "testing/win32_window_test.cc",
-    "testing/win32_window_test.h",
     "win32_dpi_utils_unittests.cc",
     "win32_flutter_window_unittests.cc",
     "win32_window_proc_delegate_manager_unittests.cc",

shell/platform/windows/testing/mock_win32_window.cc

@@ -0,0 +1,21 @@
+#include "flutter/shell/platform/windows/testing/mock_win32_window.h"
+
+namespace flutter {
+namespace testing {
+
+MockWin32Window::MockWin32Window() : Win32Window(){};
+
+MockWin32Window::~MockWin32Window() = default;
+
+UINT MockWin32Window::GetDpi() {
+  return GetCurrentDPI();
+}
+
+void MockWin32Window::InjectWindowMessage(UINT const message,
+                                          WPARAM const wparam,
+                                          LPARAM const lparam) {
+  HandleMessage(message, wparam, lparam);
+}
+
+}  // namespace testing
+}  // namespace flutter

shell/platform/windows/testing/mock_win32_window.h

@@ -0,0 +1,45 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <windowsx.h>
+
+#include "flutter/shell/platform/windows/win32_window.h"
+#include "gmock/gmock.h"
+
+namespace flutter {
+namespace testing {
+
+/// Mock for the Win32Window base class.
+class MockWin32Window : public Win32Window {
+ public:
+  MockWin32Window();
+  virtual ~MockWin32Window();
+
+  // Prevent copying.
+  MockWin32Window(MockWin32Window const&) = delete;
+  MockWin32Window& operator=(MockWin32Window const&) = delete;
+
+  // Wrapper for GetCurrentDPI() which is a protected method.
+  UINT GetDpi();
+
+  // Simulates a WindowProc message from the OS.
+  void InjectWindowMessage(UINT const message,
+                           WPARAM const wparam,
+                           LPARAM const lparam);
+
+  MOCK_METHOD1(OnDpiScale, void(unsigned int));
+  MOCK_METHOD2(OnResize, void(unsigned int, unsigned int));
+  MOCK_METHOD2(OnPointerMove, void(double, double));
+  MOCK_METHOD3(OnPointerDown, void(double, double, UINT));
+  MOCK_METHOD3(OnPointerUp, void(double, double, UINT));
+  MOCK_METHOD0(OnPointerLeave, void());
+  MOCK_METHOD0(OnSetCursor, void());
+  MOCK_METHOD1(OnText, void(const std::u16string&));
+  MOCK_METHOD4(OnKey, void(int, int, int, char32_t));
+  MOCK_METHOD2(OnScroll, void(double, double));
+  MOCK_METHOD0(OnFontChange, void());
+};
+
+}  // namespace testing
+}  // namespace flutter