Cherry-pick 6a3c566. rdar://117079338

    As a "preview" feature, content-visibility should be off by default
    https://bugs.webkit.org/show_bug.cgi?id=263428
    rdar://117079338

    Reviewed by Wenson Hsieh.

    269480@main should have turned the feature off by default, and failed to do so.

    * Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml:

    Canonical link: https://commits.webkit.org/269609@main

Identifier: 267815.439@safari-7617.1.13.10-branch

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -1009,11 +1009,11 @@ CSSContentVisibilityEnabled:
   humanReadableDescription: "Enable CSS content-visibility"
   defaultValue:
     WebKitLegacy:
-      default: true
+      default: false
     WebKit:
-      default: true
+      default: false
     WebCore:
-      default: true
+      default: false
 
 CSSCounterStyleAtRuleImageSymbolsEnabled:
   type: bool