deps,v8: fix compilation in VS2015

To compile with VS2015, the == and != methods in zone-allocator.h need
to be marked const.

This change was introduced upstream in
f9e4527f32

PR-URL: https://github.com/nodejs/node/pull/2843
Reviewed-By: rvagg - Rod Vagg <rod@vagg.org>
Reviewed-By: orangemocha - Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>

deps/v8/src/zone-allocator.h

@@ -50,10 +50,10 @@ class zone_allocator {
   }
   void destroy(pointer p) { p->~T(); }
 
-  bool operator==(zone_allocator const& other) {
+  bool operator==(zone_allocator const& other) const {
     return zone_ == other.zone_;
   }
-  bool operator!=(zone_allocator const& other) {
+  bool operator!=(zone_allocator const& other) const {
     return zone_ != other.zone_;
   }