Add "SetLowPriorityForBeacon" to the fieldtrial testing config.

Bug: 1018081
Change-Id: I7e852df70868ef52650918e1fccda6b2047b6240
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1886273Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712896}

testing/variations/fieldtrial_testing_config.json

@@ -5685,6 +5685,25 @@
             ]
         }
     ],
+    "SetLowPriorityForBeacon": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SetLowPriorityForBeacon"
+                    ]
+                }
+            ]
+        }
+    ],
     "SettingsEnforcement": [
         {
             "platforms": [