[Custom Tabs] Implement "Read It Later" Behind a Flag

testing/variations/fieldtrial_testing_config_android.json

 {
     "AndroidSessionNotifications": [
         {
             "group_name": "Disabled"
         }
     ],
     "AsyncDns": [
         {
             "group_name": "AsyncDnsA"
         },
@@ skipping 284 matching lines @@
             "params": {
                 "progress-bar-animation": "smooth"
             }
         }
     ],
     "QUIC": [
         {
             "group_name": "Enabled"
         }
     ],
+    "ReadItLater": [
+        {
+            "enable_features": [
+                "ReadItLaterInMenu"
+            ],
+            "group_name": "Enabled"
+        }
+    ],
     "ReaderModeUI": [
         {
             "group_name": "AdaBoost"
         },
         {
             "group_name": "Disabled"
         }
     ],
     "ReaderModeUIFeedback": [
         {
@@ skipping 222 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }