FirefoxDriver's object annotations should be writable

Fixes the WDBS build

Author: jleyba

javascript/firefox-driver/js/firefoxDriver.js

@@ -267,7 +267,7 @@ function injectAndExecuteScript(respond, parameters, isAsync, timer) {
         Object.defineProperty(value, '__exposedProps__', {
           enumerable: false,
           configurable: false,
-          writable: false,
+          writable: true,
           value: props
         });
       }

javascript/firefox-driver/js/moz.js

@@ -140,7 +140,7 @@ fxdriver.moz.markUnwrapped_ = function(thing) {
   Object.defineProperty(thing, '__fxdriver_unwrapped', {
     enumerable: false,
     configurable: false,
-    writable: false,
+    writable: true,
     value: true
   });
 };