Collect array buffer allocation sizes for UMA in megabytes.

src/counters.cc

Index: src/counters.cc
diff --git a/src/counters.cc b/src/counters.cc
index 996aa333d411fe08526f1f80bc00a351dabae21d..87ab983bd94f12d4ab9de32a27c84382623cada4 100644
--- a/src/counters.cc
+++ b/src/counters.cc
@@ -81,6 +81,24 @@ Counters::Counters(Isolate* isolate) {
   }
 
   static const struct {
+    MegabyteHistogram Counters::*member;
+    const char* caption;
+    int min;
+    int max;
+    int num_buckets;
+  } kMegabyteHistograms[] = {
+#define HR(name, caption, min, max, num_buckets) \
+  {&Counters::name##_, #caption, min, max, num_buckets},
+      MEGABYTE_HISTOGRAM_RANGE_LIST(HR)
+#undef HR
+  };
+  for (const auto& histogram : kMegabyteHistograms) {
+    this->*histogram.member =
+        MegabyteHistogram(histogram.caption, histogram.min, histogram.max,
+                          histogram.num_buckets, isolate);
+  }
+
+  static const struct {
     HistogramTimer Counters::*member;
     const char* caption;
     int max;
@@ -238,6 +256,10 @@ void Counters::ResetHistograms() {
   HISTOGRAM_RANGE_LIST(HR)
 #undef HR
 
+#define HR(name, caption, min, max, num_buckets) name##_.Reset();
+  MEGABYTE_HISTOGRAM_RANGE_LIST(HR)
+#undef HR
+
 #define HT(name, caption, max, res) name##_.Reset();
     HISTOGRAM_TIMER_LIST(HT)
 #undef HT