Bug 1363053 - Return JSON representation of return value r=maja_zf

According to a recent change in the WebDriver specification, we need to
return an object's JSON representation iff it exists.

The relevant specification prose change was made in
1ee4c61c11.

This patch causes return values that have a toJSON property that is a
function, to return the value of calling said function.

MozReview-Commit-ID: GpQNE9GpjCH

--HG--
extra : rebase_source : 7192bbd484cbaa4661f2442990082aefcdd1570b

testing/marionette/evaluate.js

@@ -247,7 +247,7 @@ evaluate.fromJSON = function (obj, seenEls, win, shadowRoot = undefined) {
  *     web elements.
  */
 evaluate.toJSON = function (obj, seenEls) {
-  let t = Object.prototype.toString.call(obj);
+  const t = Object.prototype.toString.call(obj);
 
   // null
   if (t == "[object Undefined]" || t == "[object Null]") {
@@ -270,6 +270,12 @@ evaluate.toJSON = function (obj, seenEls) {
     return element.makeWebElement(uuid);
   }
 
+  // custom JSON representation
+  else if (typeof obj["toJSON"] == "function") {
+    let unsafeJSON = obj.toJSON();
+    return evaluate.toJSON(unsafeJSON, seenEls);
+  }
+
   // arbitrary objects + files
   else {
     let rv = {};

testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py

@@ -49,6 +49,9 @@ class TestExecuteContent(MarionetteTestCase):
             "return typeof arguments[0] != 'undefined'", [property], sandbox=sandbox),
             "property {} is undefined".format(property))
 
+    def assert_is_web_element(self, element):
+        self.assertIsInstance(element, HTMLElement)
+
     def test_return_number(self):
         self.assertEqual(1, self.marionette.execute_script("return 1"))
         self.assertEqual(1.5, self.marionette.execute_script("return 1.5"))
@@ -330,6 +333,25 @@ class TestExecuteContent(MarionetteTestCase):
         # test inspection of arguments
         self.marionette.execute_script("__webDriverArguments.toString()")
 
+    def test_toJSON(self):
+        foo = self.marionette.execute_script("""
+            return {
+              toJSON () {
+                return "foo";
+              }
+            }""",
+            sandbox=None)
+        self.assertEqual("foo", foo)
+
+    def test_unsafe_toJSON(self):
+        el = self.marionette.execute_script("""
+            return {
+              toJSON () {
+                return document.documentElement;
+              }
+            }""",
+            sandbox=None)
+        self.assert_is_web_element(el)
 
 
 class TestExecuteChrome(WindowManagerMixin, TestExecuteContent):