Bug 1455589 - Drop margin at right end of DevTools toolbox toolbar on non-Mac platforms; r=jryans

This margin only appears to be needed on Mac hence this patch makes the setting
apply only on OSX. It also switches to using logical properties so that the
margin appears in the correct place in RTL builds.

MozReview-Commit-ID: Chp1HJcretg

--HG--
extra : rebase_source : ef16f6175628be864b002ddf8d3a318570e39942

devtools/client/themes/toolbox.css

@@ -164,8 +164,10 @@
   transform: rotate(180deg);
 }
 
-#toolbox-controls {
-  margin-right: 3px;
+/* On OSX the cursor turns into a window-resizing cursor at the edges of the
+ * window, so bring the end of the toolbox in. */
+:root[platform="mac"] #toolbox-controls {
+  margin-inline-end: 3px;
 }
 
 #toolbox-buttons-end > .devtools-separator {