Java: Implements union generation.

mojo/public/java/bindings/src/org/chromium/mojo/bindings/BindingsHelper.java

Index: mojo/public/java/bindings/src/org/chromium/mojo/bindings/BindingsHelper.java
diff --git a/mojo/public/java/bindings/src/org/chromium/mojo/bindings/BindingsHelper.java b/mojo/public/java/bindings/src/org/chromium/mojo/bindings/BindingsHelper.java
index 169edcbd79916a6bde644bc3b21536dcf62802cb..614631627468c156b447523b84a8c508a1707b8a 100644
--- a/mojo/public/java/bindings/src/org/chromium/mojo/bindings/BindingsHelper.java
+++ b/mojo/public/java/bindings/src/org/chromium/mojo/bindings/BindingsHelper.java
@@ -4,7 +4,6 @@
 
 package org.chromium.mojo.bindings;
 
-import org.chromium.mojo.bindings.Struct.DataHeader;
 import org.chromium.mojo.system.AsyncWaiter;
 import org.chromium.mojo.system.Handle;
 
@@ -36,6 +35,11 @@ public class BindingsHelper {
     public static final int POINTER_SIZE = 8;
 
     /**
+     * The size, in bytes, of a serialized union.
+     */
+    public static final int UNION_SIZE = 16;
+
+    /**
      * The header for a serialized map element.
      */
     public static final DataHeader MAP_STRUCT_HEADER = new DataHeader(24, 0);