Use timeout multiplier for Marionette-backed reftests.

timeout_multiplier is set in InternalRefTestImplementation's
constructor but not used.  Instead it appears to only be using the
test's timeout value.

According to the bug report by roc, reftests time out after 10
seconds, when it should be 40.

Thanks-to: Robert O'Callahan <roc@ocallahan.org>

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1483766
gecko-commit: 43de4e0d1745d722cbb3a6a29a838609c8d82f3c
gecko-integration-branch: mozilla-inbound
gecko-reviewers: jgraham
diff --git a/tools/wptrunner/wptrunner/executors/executormarionette.py b/tools/wptrunner/wptrunner/executors/executormarionette.py
index 9df4508..70054b5 100644
--- a/tools/wptrunner/wptrunner/executors/executormarionette.py
+++ b/tools/wptrunner/wptrunner/executors/executormarionette.py
@@ -817,11 +817,12 @@
 
     def run_test(self, test):
         references = self.get_references(test)
+        timeout = (test.timeout * 1000) * self.timeout_multiplier
         rv = self.executor.protocol.marionette._send_message("reftest:run",
                                                              {"test": self.executor.test_url(test),
                                                               "references": references,
                                                               "expected": test.expected(),
-                                                              "timeout": test.timeout * 1000})["value"]
+                                                              "timeout": timeout})["value"]
         return rv
 
     def get_references(self, node):