Set enable_same_site param for BackForwardCache in fieldtrial_testing_config.json

We're going to experiment with enable_same_site = true.

Change-Id: Id1a568fe56ab62a393f825873c061f3175a8a9f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416650Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808760}

testing/variations/fieldtrial_testing_config.json

@@ -1041,6 +1041,7 @@
                     "params": {
                         "TimeToLiveInBackForwardCacheInSeconds": "180",
                         "allowed_websites": "https://domain1.org,https://domain2.org",
+                        "enable_same_site": "true",
                         "geolocation_supported": "true",
                         "service_worker_supported": "true"
                     },