Add fieldtrial testing config for TabSwitcherLongpressMenu

This CL adds the missing field trial testing config for the
feature TabSwitcherLongpressMenu.

Bug: 998848
Change-Id: I7d4ae7a78631cafe3dbe44d708c42f645bc00d78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1822660Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/master@{#699528}

testing/variations/fieldtrial_testing_config.json

@@ -6039,6 +6039,21 @@
             ]
         }
     ],
+    "TabSwitcherLongpressMenu": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "TabSwitcherLongpressMenu"
+                    ]
+                }
+            ]
+        }
+    ],
     "TabToGTSAnimation": [
         {
             "platforms": [