Bug 1316922 - Fix border radius of add-on sdk panels on macOS. r=kmag

MozReview-Commit-ID: EyIziH3Ui2T

--HG--
extra : rebase_source : 9d89b8346aedaf927b6f28c812f6ef83e1d0fd28

addon-sdk/source/lib/sdk/panel/utils.js

@@ -255,7 +255,7 @@ function setupPanelFrame(frame) {
   frame.setAttribute("autocompleteenabled", true);
   frame.setAttribute("tooltip", "aHTMLTooltip");
   if (platform === "darwin") {
-    frame.style.borderRadius = "6px";
+    frame.style.borderRadius = "var(--arrowpanel-border-radius, 3.5px)";
     frame.style.padding = "1px";
   }
 }