Bug 1370810 - Add --parallel-stylo-traversal flag to wpt test. r=jmaher

MozReview-Commit-ID: B1hfuV51UtS

--HG--
extra : rebase_source : 03cb65ca05847aecb99b33fc990581a8121228d9

testing/mozharness/scripts/web_platform_tests.py

@@ -56,6 +56,12 @@ class WebPlatformTest(TestingMixin, MercurialScript, BlobUploadMixin, CodeCovera
             "default": False,
             "help": "Permits a software GL implementation (such as LLVMPipe) to use the GL compositor."}
          ],
+        [["--parallel-stylo-traversal"], {
+            "action": "store_true",
+            "dest": "parallel_stylo_traversal",
+            "default": False,
+            "help": "Forcibly enable parallel traversal in Stylo with STYLO_THREADS=4"}
+         ],
         [["--enable-webrender"], {
             "action": "store_true",
             "dest": "enable_webrender",
@@ -223,6 +229,8 @@ class WebPlatformTest(TestingMixin, MercurialScript, BlobUploadMixin, CodeCovera
         if self.config['enable_webrender']:
             env['MOZ_WEBRENDER'] = '1'
 
+        env['STYLO_THREADS'] = '4' if self.config['parallel_stylo_traversal'] else '1'
+
         env = self.query_env(partial_env=env, log_level=INFO)
 
         return_code = self.run_command(cmd,