Add section for SafeBrowsingUpdateFrequency in fieldtrial on win/mac/linux

testing/variations/fieldtrial_testing_config_linux.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 95 matching lines @@
         {
             "group_name": "Enabled"
         },
         {
             "group_name": "Control"
         },
         {
             "group_name": "Disabled"
         }
     ],
+    "SafeBrowsingUpdateFrequency": [
+        {
+            "group_name": "UpdateTime15m",
+            "params": {
+                "NextUpdateIntervalInMinutes": "15"
+            }
+        }
+    ],
     "SdchPersistence": [
         {
             "group_name": "Enabled"
         }
     ],
     "SyncHttpContentCompression": [
         {
             "group_name": "Enabled"
         },
         {
@@ skipping 22 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }