[GCC] Debian stable build bot broken after 250926@main

https://bugs.webkit.org/show_bug.cgi?id=240897

Reviewed by Cameron McCormack.

GCC8.3 (Debian 10) doesn't support direct brace enclosed initialization of
bitfield enums.

* Source/WebCore/platform/text/TextFlags.h: Move initialization of
  bitfield enums to constructor.

Canonical link: https://commits.webkit.org/250944@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@294785 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Source/WebCore/platform/text/TextFlags.h

@@ -65,7 +65,12 @@ struct ExpansionBehavior {
         Force
     };
 
-    ExpansionBehavior() = default;
+    ExpansionBehavior()
+        : left(Behavior::Forbid)
+        , right(Behavior::Allow)
+    {
+
+    }
 
     ExpansionBehavior(Behavior left, Behavior right)
         : left(left)
@@ -98,8 +103,9 @@ struct ExpansionBehavior {
         return { Behavior::Forbid, Behavior::Forbid };
     }
 
-    Behavior left : 2 { Behavior::Forbid };
-    Behavior right : 2 { Behavior::Allow };
+    static constexpr unsigned bitsOfKind = 2;
+    Behavior left : bitsOfKind;
+    Behavior right : bitsOfKind;
 };
 
 enum FontSynthesisValues {