[scroll-animations] add a runtime flag for Scroll-driven Animations

https://bugs.webkit.org/show_bug.cgi?id=264260

Reviewed by Tim Nguyen.

Add a runtime flag for the Scroll-driven Animations spec (https://drafts.csswg.org/scroll-animations-1/).

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

Canonical link: https://commits.webkit.org/270282@main

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -5698,6 +5698,20 @@ ScrollAnimatorEnabled:
     WebCore:
       default: false
 
+ScrollDrivenAnimationsEnabled:
+  type: bool
+  status: testable
+  category: animation
+  humanReadableName: "Scroll-driven Animations"
+  humanReadableDescription: "Enable Scroll-driven Animations"
+  defaultValue:
+    WebKitLegacy:
+      default: false
+    WebKit:
+      default: false
+    WebCore:
+      default: false
+
 ScrollToTextFragmentEnabled:
   type: bool
   status: stable