Enable LegacySymantecPKI deprecation in Finch JSON

This is already enabled by default in source, but
this updates the JSON to match the Finch config.

Bug: 881930
Change-Id: I26f637447141730e24f0a2e2c4d84b05f2cba2fa
Reviewed-on: https://chromium-review.googlesource.com/c/1278621Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599837}

testing/variations/fieldtrial_testing_config.json

@@ -2370,6 +2370,25 @@
             ]
         }
     ],
+    "LegacySymantecPKI": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_20181012",
+                    "enable_features": [
+                        "LegacySymantecPKI"
+                    ]
+                }
+            ]
+        }
+    ],
     "LocalScreenCasting": [
         {
             "platforms": [