Fork WebAppCreator task to use plain java classloader

When ant's classloader references a directory within a jar and is
run from Java 8+, it incorrectly appends a trailing slash to the
java URL instance. For WebAppCreator, this is a breaking change,
and forking this task brings it in line with the other <java> tasks
throughout the project.

With that changed, samples in the gwt release zip correctly include
build files, readmes, and some other contents.

Change-Id: I5ba2a3024e07c7f817716a4c4c428acd745405bf

samples/common.ant.xml

diff --git a/samples/common.ant.xml b/samples/common.ant.xml
index 5d0df9e..4de54d0 100755
--- a/samples/common.ant.xml
+++ b/samples/common.ant.xml
@@ -174,7 +174,7 @@
         </targetfiles>
         <sequential>
           <mkdir dir="${samples.scripts}/${sample.upper}" />
-          <java classname="com.google.gwt.user.tools.WebAppCreator" classpath="${gwt.user.jar}:${gwt.dev.jar}" failonerror="true">
+          <java classname="com.google.gwt.user.tools.WebAppCreator" classpath="${gwt.user.jar}:${gwt.dev.jar}" failonerror="true" fork="true">
             <!-- Relative path is important! Paths will be relative in final distro -->
             <sysproperty key="gwt.devjar" value="../../gwt-dev.jar" />
             <arg value="-XnoEclipse" />