Convert RemoteWebElements decoded from JSON to "known elements".

As of commit e32b83d, the decoded JSON parameters may include a WebElement. It's necessary to convert the element when a RemoteWebElement is passed as the "id" param to SwitchToFrame.

Fixes SeleniumHQ#4083.

java/server/src/org/openqa/selenium/remote/server/handler/internal/ArgumentConverter.java

@@ -22,6 +22,7 @@
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 
+import org.openqa.selenium.remote.RemoteWebElement;
 import org.openqa.selenium.remote.server.KnownElements;
 
 import java.util.List;
@@ -51,6 +52,10 @@ public Object apply(Object arg) {
       return converted;
     }
 
+    if (arg instanceof RemoteWebElement) {
+      return knownElements.get(((RemoteWebElement) arg).getId());
+    }
+
     if (arg instanceof List<?>) {
       return Lists.newArrayList(Iterables.transform((List<?>) arg, this));
     }