MacViews: Add native drop shadow to dialogs on OSX < 10.10.

ui/views/cocoa/bridged_native_widget.mm

Index: ui/views/cocoa/bridged_native_widget.mm
diff --git a/ui/views/cocoa/bridged_native_widget.mm b/ui/views/cocoa/bridged_native_widget.mm
index 24bba74dd91727e12fe250c3c02c4b1145875e10..b22e210c1fc25bcaa06cbb439a50d6364ccc9ac0 100644
--- a/ui/views/cocoa/bridged_native_widget.mm
+++ b/ui/views/cocoa/bridged_native_widget.mm
@@ -850,7 +850,14 @@ void BridgedNativeWidget::CreateLayer(ui::LayerType layer_type,
   // native shape is what's most appropriate for displaying sheets on Mac.
   if (translucent && !native_widget_mac_->IsWindowModalSheet()) {
     [window_ setOpaque:NO];
-    [window_ setBackgroundColor:[NSColor clearColor]];
+    // For Mac OS versions earlier than Yosemite, the Window server isn't able
+    // to generate a window shadow from the composited CALayer. To get around
+    // this, set an opaque background on the window and clip the window boundary
+    // in drawRect method of BridgedContentView. See crbug.com/543671.
+    if (base::mac::IsOSYosemiteOrLater())
+      [window_ setBackgroundColor:[NSColor clearColor]];
+    else
+      [window_ setBackgroundColor:[NSColor whiteColor]];
   }
 
   UpdateLayerProperties();