Add fieldtrial_testing_config for new CSD model

The histograms show no major regressions on Canary/Dev. We'll need the
increased data volume from Beta to validate the model performance
improvements. Before doing so, enable the new model version in field
trials. This should have no effect, since this parameter chooses a
model version in communication with remote SB servers.

Bug: 1054056
Change-Id: Ifa10a303d880cb05c5176a8813b2bece06a992f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216389
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774394}

testing/variations/fieldtrial_testing_config.json

@@ -1481,6 +1481,24 @@
             ]
         }
     ],
+    "ClientSideDetectionModel": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "params": {
+                        "ModelNum": "4"
+                    }
+                }
+            ]
+        }
+    ],
     "ClipboardMaximumAge": [
         {
             "platforms": [