Add historgram for number of GC needed to collect a detached context.

BUG=

Review URL: https://codereview.chromium.org/934773002

Cr-Commit-Position: refs/heads/master@{#26690}

src/counters.h

@@ -360,6 +360,7 @@ class AggregatedHistogramTimerScope {
 
 #define HISTOGRAM_RANGE_LIST(HR)                                              \
   /* Generic range histograms */                                              \
+  HR(detached_context_age_in_gc, V8.DetachedContextAgeInGC, 0, 20, 21)        \
   HR(gc_idle_time_allotted_in_ms, V8.GCIdleTimeAllottedInMS, 0, 10000, 101)   \
   HR(gc_idle_time_limit_overshot, V8.GCIdleTimeLimit.Overshot, 0, 10000, 101) \
   HR(gc_idle_time_limit_undershot, V8.GCIdleTimeLimit.Undershot, 0, 10000,    \

src/flag-definitions.h

@@ -620,8 +620,11 @@ DEFINE_BOOL(trace_incremental_marking, false,
             "trace progress of the incremental marking")
 DEFINE_BOOL(track_gc_object_stats, false,
             "track object counts and memory usage")
-DEFINE_BOOL(track_detached_contexts, false,
+DEFINE_BOOL(track_detached_contexts, true,
             "track native contexts that are expected to be garbage collected")
+DEFINE_BOOL(trace_detached_contexts, false,
+            "trace native contexts that are expected to be garbage collected")
+DEFINE_IMPLICATION(trace_detached_contexts, track_detached_contexts)
 #ifdef VERIFY_HEAP
 DEFINE_BOOL(verify_heap, false, "verify heap pointers before and after GC")
 #endif

src/isolate.cc

@@ -2598,21 +2598,25 @@ 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);
     }
   }
-  PrintF("%d detached contexts are collected out of %d\n", length - new_length,
-         length);
-  for (int i = 0; i < new_length; i += 2) {
-    int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
-    WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
-    if (mark_sweeps > 3) {
-      PrintF("detached context 0x%p\n survived %d GCs (leak?)\n",
-             static_cast<void*>(cell->value()), mark_sweeps);
+  if (FLAG_trace_detached_contexts) {
+    PrintF("%d detached contexts are collected out of %d\n",
+           length - new_length, length);
+    for (int i = 0; i < new_length; i += 2) {
+      int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
+      WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
+      if (mark_sweeps > 3) {
+        PrintF("detached context 0x%p\n survived %d GCs (leak?)\n",
+               static_cast<void*>(cell->value()), mark_sweeps);
+      }
     }
   }
-  if (length == new_length) {
+  if (new_length == 0) {
     heap()->set_detached_contexts(heap()->empty_fixed_array());
-  } else {
+  } else if (new_length < length) {
     heap()->RightTrimFixedArray<Heap::FROM_GC>(*detached_contexts,
                                                length - new_length);
   }