Render opaque border with no shadow for web contents modal dialogs under Views/Win32

ui/views/bubble/bubble_frame_view.cc

Index: ui/views/bubble/bubble_frame_view.cc
diff --git a/ui/views/bubble/bubble_frame_view.cc b/ui/views/bubble/bubble_frame_view.cc
index c1ae46d55422c8b9ba96b89ec370c80d70adc0c9..63dcac14bf70d0f98731d24a80081fb88f121abf 100644
--- a/ui/views/bubble/bubble_frame_view.cc
+++ b/ui/views/bubble/bubble_frame_view.cc
@@ -9,6 +9,7 @@
 #include "grit/ui_resources.h"
 #include "ui/base/hit_test.h"
 #include "ui/base/resource/resource_bundle.h"
+#include "ui/gfx/path.h"
 #include "ui/gfx/screen.h"
 #include "ui/views/bubble/bubble_border.h"
 #include "ui/views/controls/button/label_button.h"
@@ -96,7 +97,42 @@ int BubbleFrameView::NonClientHitTest(const gfx::Point& point) {
 }
 
 void BubbleFrameView::GetWindowMask(const gfx::Size& size,
-                                    gfx::Path* window_mask) {}
+                                    gfx::Path* window_mask) {
+  if (bubble_border_->shadow() != BubbleBorder::NO_SHADOW_OPAQUE_BORDER)
+    return;
+
+  // Use a window mask roughly matching the border in the image assets.
+
+  // Stroke size in pixels of borders in image assets.
+  static const int kBorderStrokeSize = 1;
+
+  gfx::Insets border_insets = bubble_border_->GetInsets();
+  SkRect rect = {border_insets.left() - kBorderStrokeSize,
+                 border_insets.top() - kBorderStrokeSize,
+                 size.width() - border_insets.right() + kBorderStrokeSize,
+                 size.height() - border_insets.bottom() + kBorderStrokeSize};
+
+  // Approximate rounded corners matching the border.
+  SkPoint polygon[] = {
+    {rect.left() + 2, rect.top()},
+    {rect.left() + 1, rect.top() + 1},
+    {rect.left(), rect.top() + 2},
+
+    {rect.left(), rect.bottom() - 3},
+    {rect.left() + 1, rect.bottom() - 2},
+    {rect.left() + 2, rect.bottom()},
+
+    {rect.right() - 3, rect.bottom()},
+    {rect.right() - 1, rect.bottom() - 2},
+    {rect.right(), rect.bottom() - 3},
+
+    {rect.right(), rect.top() + 2},
+    {rect.right() - 1, rect.top() + 1},
+    {rect.right() - 2, rect.top()}
+  };
+
+  window_mask->addPoly(polygon, sizeof(polygon)/sizeof(polygon[0]), true);
+}
 
 void BubbleFrameView::ResetWindowControls() {}