Bug 1368264 - Have mach pick up geckodriver from dist/bin r=jgraham

MozReview-Commit-ID: Fs9F6o7jNDP

--HG--
extra : rebase_source : 828d8aab4cba6c951768bbf3e6de5a55af889918

testing/web-platform/mach_commands.py

@@ -65,6 +65,9 @@ class WebPlatformTestsRunner(MozbuildObject):
 
         kwargs["capture_stdio"] = True
 
+        if kwargs["webdriver_binary"] is None:
+            kwargs["webdriver_binary"] = self.get_binary_path("geckodriver")
+
         kwargs = wptcommandline.check_args(kwargs)
 
     def setup_kwargs_wptrun(self, kwargs):