Create ConnectionStateHandler instance to enable overriding (#19017)

Change-Id: Idef9f42d4db109f236bc7b5e756c9e7441a618fc

client/src/com/vaadin/Vaadin.gwt.xml

@@ -80,6 +80,12 @@
         <when-property-is value="ie10" name="user.agent" />
     </replace-with>
 
+    <replace-with
+        class="com.vaadin.client.communication.DefaultConnectionStateHandler">
+        <when-type-is
+            class="com.vaadin.client.communication.ConnectionStateHandler" />
+    </replace-with>
+
     <!-- Make Sass linking available -->
     <define-linker name="sass"
         class="com.vaadin.sass.linker.SassLinker" />

client/src/com/vaadin/client/ApplicationConnection.java

@@ -45,7 +45,6 @@ import com.vaadin.client.ApplicationConfiguration.ErrorMessage;
 import com.vaadin.client.ResourceLoader.ResourceLoadEvent;
 import com.vaadin.client.ResourceLoader.ResourceLoadListener;
 import com.vaadin.client.communication.ConnectionStateHandler;
-import com.vaadin.client.communication.DefaultConnectionStateHandler;
 import com.vaadin.client.communication.Heartbeat;
 import com.vaadin.client.communication.MessageHandler;
 import com.vaadin.client.communication.MessageSender;
@@ -360,8 +359,7 @@ public class ApplicationConnection implements HasHandlers {
         tooltip = GWT.create(VTooltip.class);
         loadingIndicator = GWT.create(VLoadingIndicator.class);
         serverRpcQueue = GWT.create(ServerRpcQueue.class);
-        connectionStateHandler = GWT
-                .create(DefaultConnectionStateHandler.class);
+        connectionStateHandler = GWT.create(ConnectionStateHandler.class);
         messageHandler = GWT.create(MessageHandler.class);
         messageSender = GWT.create(MessageSender.class);
     }