Bug 1399805 - fix the default path of fix_macosx_stack.py for running wpt tests. r=jgraham

We try to find fix_macosx_stack.py on the wrong path, which leads us can't run
wpt tests without setting stack fix directory manually (--stackfix-dir).
In this patch, we fix this issue by pointing the path to @topobjdir/dist/bin.

MozReview-Commit-ID: 8JzWWgVM6fM

--HG--
extra : rebase_source : a7822998d7668b976284852bdca86e70d615b611

testing/web-platform/mach_commands.py

@@ -46,8 +46,7 @@ class WebPlatformTestsRunnerSetup(MozbuildObject):
             kwargs["certutil_binary"] = self.get_binary_path('certutil')
 
         if kwargs["stackfix_dir"] is None:
-            kwargs["stackfix_dir"] = os.path.split(
-                self.get_binary_path(validate_exists=False))[0]
+            kwargs["stackfix_dir"] = self.bindir
 
         here = os.path.split(__file__)[0]