Fix sampling of V8.DetachedContextAgeInGC histogram.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 8c3fce930ed6732c281a46ed83cd30a915a52b39..7319af41c181d6ea8f89251e9f5e1132a97c28b8 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2598,9 +2598,8 @@ void Isolate::CheckDetachedContextsAfterGC() {
       detached_contexts->set(new_length, Smi::FromInt(mark_sweeps + 1));
       detached_contexts->set(new_length + 1, cell);
       new_length += 2;
-    } else {
-      counters()->detached_context_age_in_gc()->AddSample(mark_sweeps);

[ulan, 2015/02/18 13:14:33]

We need to sample all detached contexts, not only the collected ones because if
there is a memory leak then the leaked context will never be collected.

     }
+    counters()->detached_context_age_in_gc()->AddSample(mark_sweeps + 1);
   }
   if (FLAG_trace_detached_contexts) {
     PrintF("%d detached contexts are collected out of %d\n",