Enable the CheckOfflineCapability flag in a field trial testing config.

The field trial testing config will be applied to Chrome performance
bots and Chrome browser_tests. This CL enables the
CheckOfflineCapability flag for Finch experiment.

Bug: 965802
Change-Id: Ie885c727a659711baa8a93553b26d2269631ddd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2235580Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Asami Doi <asamidoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782790}

testing/variations/fieldtrial_testing_config.json

@@ -4352,6 +4352,25 @@
             ]
         }
     ],
+    "OfflineCapabilityCheckForInstallablePrompt": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CheckOfflineCapability"
+                    ]
+                }
+            ]
+        }
+    ],
     "OfflineIndicator": [
         {
             "platforms": [