WebViewOriginCheckForStreamReader: enable it in variation tests

Update fieldtrial_testing_config.json to enable it in variation
tests. This is for rolling it out in the stable.

Bug: 1096677
Change-Id: Ie37c3ab339a59b591dda2907d46314c2d4f490c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342229
Auto-Submit: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796110}

testing/variations/fieldtrial_testing_config.json

@@ -8293,6 +8293,21 @@
             ]
         }
     ],
+    "WebViewOriginCheckForStreamReader": [
+        {
+            "platforms": [
+                "android_webview"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "WebViewOriginCheckForStreamReader"
+                    ]
+                }
+            ]
+        }
+    ],
     "WindowsHybridSpellCheck": [
         {
             "platforms": [