Change RemoteServiceServlet so gwt.codeserver.port works with more subclasses.

In many subclasses of RemoteServiceServlet, the doGetSerializationPolicy()
method is overridden so that it doesn't call super. As a result, setting
gwt.codeserver.port has no effect. To avoid this, I moved the calling code
into getSerializationPolicy(), which is a final method.

A subclass can still disable fetching serialization policies from
the code server by overriding getCodeServerPolicyUrl() to return 0,
but this won't happen by accident.

Change-Id: Ib47db371b78eabb1a139948744279b36aa306bc1
Review-Link: https://gwt-review.googlesource.com/#/c/2360/

Review by: mdempsky@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11576 8db76d5a-ed1c-0410-87a9-c151d255dfc7