Build is broken on x86_64 due to narrowing warning treated as an error.
This patch is a workaround to avoid this warning. Correction should be
placed in generated code V8DOMConfiguration::ConstantConfiguration::value

BUG=https://crosswalk-project.org/jira/browse/XWALK-917

Signed-off-by: Yannick GICQUEL <yannick.gicquel@open.eurogiciel.org>