Add field trial test config for LayoutInstabilityMoveDistance.

This feature has already been enabled by default on trunk, but
I'm adding this test config to satisfy the field trial lint message
that requests it.

Bug: 981338
Change-Id: I35781ee8919d8f7314164b81e45c7f560a9fc1df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692971Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675731}

testing/variations/fieldtrial_testing_config.json

@@ -2701,6 +2701,25 @@
             ]
         }
     ],
+    "LayoutInstabilityMoveDistance": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "LayoutInstabilityMoveDistance"
+                    ]
+                }
+            ]
+        }
+    ],
     "LazyLoad": [
         {
             "platforms": [