Update V8 to r3431 as required by WebKit r51976.

Change-Id: I567392c3f8c0a0d5201a4249611ac4ccf468cd5b
diff --git a/src/regexp-stack.h b/src/regexp-stack.h
index 319ab28..fbaa6fb 100644
--- a/src/regexp-stack.h
+++ b/src/regexp-stack.h
@@ -68,7 +68,9 @@
   static Address EnsureCapacity(size_t size);
 
   // Thread local archiving.
-  static size_t ArchiveSpacePerThread() { return sizeof(thread_local_); }
+  static int ArchiveSpacePerThread() {
+    return static_cast<int>(sizeof(thread_local_));
+  }
   static char* ArchiveStack(char* to);
   static char* RestoreStack(char* from);
   static void FreeThreadResources() { thread_local_.Free(); }