Define illegal zone_allocator constructor only for MSVS

It turns out that the default constructor of allocators used in
standard containers is still needed in MSVS 2015.
This CL defines the constructor only when compiling on windows.

R=bbudge@chromium.org

Review-Url: https://codereview.chromium.org/2708593004
Cr-Commit-Position: refs/heads/master@{#43341}

src/zone/zone-allocator.h

@@ -26,8 +26,10 @@ class zone_allocator {
     typedef zone_allocator<O> other;
   };
 
-  // TODO(bbudge) Remove when V8 updates to MSVS 2015. See crbug.com/603131.
+#ifdef V8_CC_MSVC
+  // MSVS unfortunately requires the default constructor to be defined.
   zone_allocator() : zone_(nullptr) { UNREACHABLE(); }
+#endif
   explicit zone_allocator(Zone* zone) throw() : zone_(zone) {}
   explicit zone_allocator(const zone_allocator& other) throw()
       : zone_(other.zone_) {}