WheelEventRegions: Enable for field_trial_config testing

Bug: 841364
Change-Id: Ibacd58d44e208aa42e85d964c8344519de4e88f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594014
Auto-Submit: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837598}

testing/variations/fieldtrial_testing_config.json

@@ -5285,7 +5285,7 @@
             ],
             "experiments": [
                 {
-                    "name": "Enabled_20201211",
+                    "name": "Disabled",
                     "disable_features": [
                         "PaintHoldingCrossOrigin"
                     ]
@@ -8591,6 +8591,26 @@
             ]
         }
     ],
+    "WheelEventRegions": [
+        {
+            "platforms": [
+                "android",
+                "android_weblayer",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_20201211",
+                    "enable_features": [
+                        "WheelEventRegions"
+                    ]
+                }
+            ]
+        }
+    ],
     "WindowsNativeSpellChecker": [
         {
             "platforms": [