Fix missing shcore.dll error on Windows 7 (flutter#30699)

Author: cbracken

third_party/accessibility/ax/BUILD.gn

@@ -99,7 +99,6 @@ source_set("ax") {
     ]
     libs = [
       "oleacc.lib",
-      "shcore.lib",
       "uiautomationcore.lib",
     ]
   }

third_party/accessibility/base/win/display.cc

@@ -7,11 +7,75 @@
 namespace base {
 namespace win {
 
+namespace {
+
+template <typename T>
+bool AssignProcAddress(HMODULE comBaseModule, const char* name, T*& outProc) {
+  outProc = reinterpret_cast<T*>(GetProcAddress(comBaseModule, name));
+  return *outProc != nullptr;
+}
+
+// Helper class for supporting display scale factor lookup across Windows
+// versions, with fallbacks where these lookups are unavailable.
+class ScaleHelperWin32 {
+ public:
+  ScaleHelperWin32();
+  ~ScaleHelperWin32();
+
+  /// Returns the scale factor for the specified monitor. Sets |scale| to
+  /// SCALE_100_PERCENT if the API is not available.
+  HRESULT GetScaleFactorForMonitor(HMONITOR hmonitor,
+                                   DEVICE_SCALE_FACTOR* scale) const;
+
+ private:
+  using GetScaleFactorForMonitor_ =
+      HRESULT __stdcall(HMONITOR hmonitor, DEVICE_SCALE_FACTOR* scale);
+
+  GetScaleFactorForMonitor_* get_scale_factor_for_monitor_ = nullptr;
+
+  HMODULE shlib_module_ = nullptr;
+  bool scale_factor_for_monitor_supported_ = false;
+};
+
+ScaleHelperWin32::ScaleHelperWin32() {
+  if ((shlib_module_ = LoadLibraryA("Shcore.dll")) != nullptr) {
+    scale_factor_for_monitor_supported_ =
+        AssignProcAddress(shlib_module_, "GetScaleFactorForMonitor",
+                          get_scale_factor_for_monitor_);
+  }
+}
+
+ScaleHelperWin32::~ScaleHelperWin32() {
+  if (shlib_module_ != nullptr) {
+    FreeLibrary(shlib_module_);
+  }
+}
+
+HRESULT ScaleHelperWin32::GetScaleFactorForMonitor(
+    HMONITOR hmonitor,
+    DEVICE_SCALE_FACTOR* scale) const {
+  if (hmonitor == nullptr || scale == nullptr) {
+    return E_INVALIDARG;
+  }
+  if (!scale_factor_for_monitor_supported_) {
+    *scale = SCALE_100_PERCENT;
+    return S_OK;
+  }
+  return get_scale_factor_for_monitor_(hmonitor, scale);
+}
+
+ScaleHelperWin32* GetHelper() {
+  static ScaleHelperWin32* helper = new ScaleHelperWin32();
+  return helper;
+}
+
+}  // namespace
+
 float GetScaleFactorForHWND(HWND hwnd) {
   HMONITOR monitor = MonitorFromWindow(hwnd, MONITOR_DEFAULTTONEAREST);
-  DEVICE_SCALE_FACTOR scale_factor = DEVICE_SCALE_FACTOR_INVALID;
-  if (SUCCEEDED(GetScaleFactorForMonitor(monitor, &scale_factor))) {
-    return ScaleFactorToFloat(scale_factor);
+  DEVICE_SCALE_FACTOR scale = DEVICE_SCALE_FACTOR_INVALID;
+  if (SUCCEEDED(GetHelper()->GetScaleFactorForMonitor(monitor, &scale))) {
+    return ScaleFactorToFloat(scale);
   }
   return 1.0f;
 }