Enable ParkableStringsToDisk in fieldtrial_testing_config.json.

A beta experiment has started, align the testing configuration with the
experiment.

Bug: 1029320
Change-Id: Ib1439e056cabf9004f7bf566ebc2060eb1989f01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340047Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795491}

testing/variations/fieldtrial_testing_config.json

@@ -5143,6 +5143,25 @@
             ]
         }
     ],
+    "ParkableStringsToDisk": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "ParkableStringsToDisk",
+                    "enable_features": [
+                        "ParkableStringsToDisk"
+                    ]
+                }
+            ]
+        }
+    ],
     "PartitionConnectionsByNetworkIsolationKey": [
         {
             "platforms": [