Set up field trial test for sending sampled pings.

Adding field trial entry for sending sampled pings
for allowlist domains to Safe Browsing.

Bug: 953106
Change-Id: Ifdcbf0d80a026667f7f78a39e7123fdc39d76ba8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894247
Commit-Queue: Bettina Dea <bdea@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711962}

testing/variations/fieldtrial_testing_config.json

@@ -5359,6 +5359,24 @@
             ]
         }
     ],
+    "SafeBrowsingSendSampledPingsForAllowlistDomain": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SafeBrowsingSendSampledPingsForAllowlistDomain"
+                    ]
+                }
+            ]
+        }
+    ],
     "SafeBrowsingSuspiciousSiteTrigger": [
         {
             "platforms": [