[Sync::Scrypt] Enable encryption in fieldtrial_testing_config.json.

Enable scrypt key derivation when setting sync custom passphrase in
testing/variations/fieldtrial_testing_config.json.

Bug: 875901
Change-Id: Idcfb76d8e1c1977875bd401520e665ffb0010399
Reviewed-on: https://chromium-review.googlesource.com/c/1483077
Auto-Submit: vitaliii <vitaliii@chromium.org>
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635246}

testing/variations/fieldtrial_testing_config.json

@@ -4818,6 +4818,26 @@
             ]
         }
     ],
+    "SyncScryptCustomPassphrase": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "ios",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SyncUseScryptForNewCustomPassphrases"
+                    ]
+                }
+            ]
+        }
+    ],
     "SyncUssBookmarks'": [
         {
             "platforms": [