Add temporary iOS platform to aid W3C migration (#4559)

In some cases we cannot consider platform and platformName to be
equivalent. For example, SauceLabs uses platformName and not
platform to run Appium tests. If we override platformName (iOS) with
platform (Mac, because no iOS platform exists), we generate a set of
invalid capabilities to actually run the browser test.

Fixes #4557

java/client/src/org/openqa/selenium/Platform.java

@@ -166,6 +166,16 @@ public Platform family() {
     }
   },
 
+  /**
+   * Provide a temporary workaround for an issue where platformName was being overridden by
+   * platform for external providers.
+   */
+  @Deprecated
+  IOS("iOS") {
+    @Override
+    public Platform family() { return MAC; }
+  },
+
   /**
    * Never returned, but can be used to request a browser running on any operating system.
    */