Bug 1411281 - Add equality test for webdriver.Element r=jgraham

When comparing two instances of webdriver.Element we want to first
check the type, to make sure the "id" attribute is present, then
compare the web element reference UUIDs.  These are supposed to be
unique across all browsing contexts.

MozReview-Commit-ID: 68PUBQxPdQ5

--HG--
extra : rebase_source : f0c52efd0db3a13ea1b90dc6c058ebee2de76e1d

testing/web-platform/tests/tools/webdriver/webdriver/client.py

@@ -625,6 +625,10 @@ class Element(object):
         assert id not in self.session._element_cache
         self.session._element_cache[self.id] = self
 
+    def __eq__(self, other):
+        return isinstance(other, Element) and self.id == other.id \
+                and self.session == other.session
+
     def send_element_command(self, method, uri, body=None):
         url = "element/%s/%s" % (self.id, uri)
         return self.session.send_session_command(method, url, body)