Add WebSocketHandshakeReuseConnection to fieldtrial testing config

Enable WebSocketHandshakeReuseConnection in
fieldtrial_testing_config.json so that it gets testing coverage on the
bots before it becomes the default.

BUG=423609

Change-Id: I692aa12c5685d61a47b26aa014d10db7bb91d98a
Reviewed-on: https://chromium-review.googlesource.com/1151182Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580484}

testing/variations/fieldtrial_testing_config.json

@@ -4858,6 +4858,26 @@
             ]
         }
     ],
+    "WebSocketHandshakeReuseConnection": [
+        {
+            "platforms": [
+                "windows",
+                "mac",
+                "chromeos",
+                "linux",
+                "android",
+                "android_webview"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "WebSocketHandshakeReuseConnection"
+                    ]
+                }
+            ]
+        }
+    ],
     "WheelScrollLatchingAndAsyncWheelEvents": [
         {
             "platforms": [