[WV] Add test config for WebViewPartitionedCookiesExcluded

This adds a JSON config entry for WebViewPartitionedCookiesExcluded so
that A/B experimentation can occur for this feature. This flag was added
in `[WV] Determine if partitioned cookies are not sent to apps. |
https://chromium-review.googlesource.com/c/chromium/src/+/6237015`.

Test: N/A
Bug: 41496912
Change-Id: I29c0a210364734675331a3bf180ee0b33b7c45cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6240130
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Adam Walls <avvall@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1417090}

testing/variations/fieldtrial_testing_config.json

@@ -26194,6 +26194,21 @@
             ]
         }
     ],
+    "WebViewPartitionedCookiesExcluded": [
+        {
+            "platforms": [
+                "android_webview"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "WebViewPartitionedCookiesExcluded"
+                    ]
+                }
+            ]
+        }
+    ],
     "WebViewPrefetchNativeLibrary": [
         {
             "platforms": [