Enable lh/rlh unit support

https://bugs.webkit.org/show_bug.cgi?id=246522
rdar://101172926

Reviewed by Darin Adler.

Bug 211356 disabled it because of bug 211351, but the linked CSSWG issue says WebKit behavior is correct (Rune & Emilio both confirmed):

> RESOLVED: browser "fixups" of line height on form controls do *not* affect lh/rlh on the element itself. Effect of fixups on lh/rlh of descendants is explicitly undefined.

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

Canonical link: https://commits.webkit.org/255540@main

Source/WTF/Scripts/Preferences/WebPreferencesInternal.yaml

@@ -608,13 +608,15 @@ LegacyOverflowScrollingTouchEnabled:
 LineHeightUnitsEnabled:
   type: bool
   humanReadableName: "lh / rlh units"
-  humanReadableDescription: "Enable the lh and lhr units"
+  humanReadableDescription: "Enable the lh and rlh units"
   webcoreBinding: DeprecatedGlobalSettings
   defaultValue:
     WebKitLegacy:
-      default: false
+      default: true
     WebKit:
-      default: false
+      default: true
+    WebCore:
+      default: true
 
 LiveRangeSelectionEnabled:
   type: bool