Fixes mozilla-mobile#2341: Gives onCancelEditing default implementation

Author: sblatz

components/concept/toolbar/src/main/java/mozilla/components/concept/toolbar/Toolbar.kt

@@ -129,7 +129,7 @@ interface Toolbar {
         /**
          * Fired when the user presses the back button while in edit mode.
          */
-        fun onCancelEditing(): Boolean
+        fun onCancelEditing(): Boolean = true
 
         /**
          * Fired when the toolbar switches back to display mode.