GWT compiler is launched as a separate java process,
and it's better to implicitly start headless execution
Change-Id: Ied4e8da5e0c2edc436e4b25602bf8e5aefd15f23
<!-- TODO: Do we need these? -->
<!-- <jvmarg value="-Xss8M" /> -->
<!-- <jvmarg value="-XX:MaxPermSize=256M" /> -->
- <!-- <jvmarg value="-Djava.awt.headless=true" /> -->
+ <jvmarg value="-Djava.awt.headless=true" />
<argument>-Dgwt.persistentunitcachedir=${project.build.directory}/compile</argument>
<argument>-DvFailIfNotSerializable=true</argument>