Fix compilation with clang 5.1 on OS X.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/250873003

Patch from Benedikt Meurer <bmeurer@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20970 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/assembler.cc

@@ -220,6 +220,7 @@ PlatformFeatureScope::PlatformFeatureScope(Isolate* isolate, CpuFeature f)
   ASSERT(Serializer::enabled());
   uint64_t mask = static_cast<uint64_t>(1) << f;
   CpuFeatures::cross_compile_ |= mask;
+  USE(isolate_);
 }
 
 

src/bootstrapper.cc

@@ -2506,6 +2506,7 @@ class NoTrackDoubleFieldsForSerializerScope {
       // immutable by the serializer.
       FLAG_track_double_fields = false;
     }
+    USE(isolate_);
   }
   ~NoTrackDoubleFieldsForSerializerScope() {
     if (Serializer::enabled()) {