[FragmentItem] Add to fieldtrial_testing_config.json

crbug.com/1100070 says this is needed to enable FragmentItem
on test bots.

Bug: 1100070
Change-Id: If0aa58bcc95fd4fbb6338f89c207cbfc31da3251
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274620Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783849}

testing/variations/fieldtrial_testing_config.json

@@ -2828,6 +2828,26 @@
             ]
         }
     ],
+    "FragmentItem": [
+        {
+            "platforms": [
+                "android",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_20200615",
+                    "enable_features": [
+                        "FragmentItem"
+                    ]
+                }
+            ]
+        }
+    ],
     "FreezeBackgroundTabOnNetworkIdle": [
         {
             "platforms": [