Start testing BillingInterstitial on the test bots.

Bug: 867518
Change-Id: I16891cc486f3d884229482b8c4d90309f9994b43
Reviewed-on: https://chromium-review.googlesource.com/c/1327810Reviewed-by: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607028}

testing/variations/fieldtrial_testing_config.json

@@ -4100,6 +4100,25 @@
             ]
         }
     ],
+    "SafeBrowsingBillingInterstitial": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows",
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "BillingInterstitial"
+                    ]
+                }
+            ]
+        }
+    ],
     "SafeBrowsingScoutTransitionStudy": [
         {
             "platforms": [