Enable kContextMenuElementPostMessage feature by default.

Bug: 228355
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I9177b7841982999299f31084ce3b042aefd59fdc
Reviewed-on: https://chromium-review.googlesource.com/942027Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561943}

ios/web/features.mm

@@ -8,7 +8,7 @@ namespace web {
 namespace features {
 
 const base::Feature kContextMenuElementPostMessage{
-    "ContextMenuElementPostMessage", base::FEATURE_DISABLED_BY_DEFAULT};
+    "ContextMenuElementPostMessage", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kSlimNavigationManager{"SlimNavigationManager",
                                            base::FEATURE_DISABLED_BY_DEFAULT};