Mac: Take alpha channel into account when reading system colors in Blink

Bug: 898278
Change-Id: I24ae7ef4e8accd54244dfd09721fa376dadfba34
Reviewed-on: https://chromium-review.googlesource.com/c/1305498Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604111}

third_party/blink/renderer/core/layout/layout_theme_mac.mm

@@ -267,10 +267,11 @@ static RGBA32 ConvertNSColorToColor(NSColor* color) {
   NSColor* color_in_color_space = ColorInColorSpace(color);
   if (color_in_color_space) {
     static const double kScaleFactor = nextafter(256.0, 0.0);
-    return MakeRGB(
+    return MakeRGBA(
         static_cast<int>(kScaleFactor * [color_in_color_space redComponent]),
         static_cast<int>(kScaleFactor * [color_in_color_space greenComponent]),
-        static_cast<int>(kScaleFactor * [color_in_color_space blueComponent]));
+        static_cast<int>(kScaleFactor * [color_in_color_space blueComponent]),
+        static_cast<int>(kScaleFactor * [color_in_color_space alphaComponent]));
   }
 
   // This conversion above can fail if the NSColor in question is an