Increase heap size used for building javadoc.

Without this change, an OutOfMemoryError exception is thrown during
clean builds.

Review by: spoon@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8334 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/doc/build.xml b/doc/build.xml
index 77416f5..9237313 100644
--- a/doc/build.xml
+++ b/doc/build.xml
@@ -84,7 +84,7 @@
       <sequential>
         <echo>Building javadoc</echo>
         <java classpathref="DOC_PATH" classname="com.google.doctool.custom.GWTJavaDoclet" fork="yes" failonerror="true">
-          <jvmarg value="-Xmx256M" />
+          <jvmarg value="-Xmx512M" />
           <arg value="-quiet" />
           <arg value="-notimestamp" />
           <arg value="-source" />