Make CSSUnprefixedBackdropFilterEnabled an embedder feature.

https://bugs.webkit.org/show_bug.cgi?id=269537
<rdar://123058619>

Reviewed by Cameron McCormack.

We want this setting to be conditionally enabled at compile time, and only 'embedder' status allows varying default values.

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

Canonical link: https://commits.webkit.org/274863@main

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -1511,20 +1511,20 @@ CSSTypedOMEnabled:
 
 CSSUnprefixedBackdropFilterEnabled:
   type: bool
-  status: testable
+  status: embedder
   category: css
   humanReadableName: "CSS Unprefixed Backdrop Filter"
   humanReadableDescription: "Enable unprefixed backdrop-filter CSS property"
   defaultValue:
     WebKitLegacy:
       "ENABLE(UNPREFIXED_BACKDROP_FILTER)": true
-      default: false
+      default: true
     WebKit:
       "ENABLE(UNPREFIXED_BACKDROP_FILTER)": true
-      default: false
+      default: true
     WebCore:
       "ENABLE(UNPREFIXED_BACKDROP_FILTER)": true
-      default: false
+      default: true
 
 CSSWordBreakAutoPhraseEnabled:
   type: bool