[v8] Add Mac to script streaming field trial

Bug: 866868
Change-Id: I2c86c688d6d3df2a4506876724ba7c5fbc4290ad
Reviewed-on: https://chromium-review.googlesource.com/c/1347360Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612613}

testing/variations/fieldtrial_testing_config.json

@@ -4341,6 +4341,7 @@
                 "android",
                 "chromeos",
                 "linux",
+                "mac",
                 "windows"
             ],
             "experiments": [