Collect array buffer allocation sizes for UMA in megabytes.

src/counters.h

Index: src/counters.h
diff --git a/src/counters.h b/src/counters.h
index e5e60fe5ade50727f92065a155a590fecfd57d00..4a59e59c8f7473f0a0a94049c7916ce742b4f73d 100644
--- a/src/counters.h
+++ b/src/counters.h
@@ -228,6 +228,21 @@ class Histogram {
   Isolate* isolate_;
 };
 
+// A histogram base on megabytes. Use method add() to add a value to
+// the table.
+class MegabyteHistogram : public Histogram {
+ public:
+  MegabyteHistogram() {}
+  MegabyteHistogram(const char* name, int min, int max, int num_buckets,
+                    Isolate* isolate)
+      : Histogram(name, min, max, num_buckets, isolate) {}
+
+  void Add(size_t size) {
+    constexpr size_t kBitsInMegabyte = 20;
+    AddSample(size >> kBitsInMegabyte);
+  }
+};
+
 // A HistogramTimer allows distributions of results to be created.
 class HistogramTimer : public Histogram {
  public:
@@ -928,9 +943,11 @@ class RuntimeCallTimerScope {
   HR(wasm_functions_per_asm_module, V8.WasmFunctionsPerModule.asm, 1, 100000, \
      51)                                                                      \
   HR(wasm_functions_per_wasm_module, V8.WasmFunctionsPerModule.wasm, 1,       \
-     100000, 51)                                                              \
-  HR(array_buffer_big_allocations, V8.ArrayBufferBigAllocations, 1, 32, 32)   \
-  HR(array_buffer_new_size_failures, V8.ArrayBufferNewSizeFailures, 1, 32, 32)
+     100000, 51)
+
+#define MEGABYTE_HISTOGRAM_RANGE_LIST(HR)                                     \
+  HR(array_buffer_big_allocations, V8.ArrayBufferBigAllocations, 1, 4096, 13) \
+  HR(array_buffer_new_size_failures, V8.ArrayBufferNewSizeFailures, 1, 4096, 13)
 
 #define HISTOGRAM_TIMER_LIST(HT)                                               \
   /* Garbage collection timers. */                                             \
@@ -1168,6 +1185,11 @@ class Counters {
   HISTOGRAM_RANGE_LIST(HR)
 #undef HR
 
+#define HR(name, caption, min, max, num_buckets) \
+  MegabyteHistogram* name() { return &name##_; }
+  MEGABYTE_HISTOGRAM_RANGE_LIST(HR)
+#undef HR
+
 #define HT(name, caption, max, res) \
   HistogramTimer* name() { return &name##_; }
   HISTOGRAM_TIMER_LIST(HT)
@@ -1277,6 +1299,10 @@ class Counters {
   HISTOGRAM_RANGE_LIST(HR)
 #undef HR
 
+#define HR(name, caption, min, max, num_buckets) MegabyteHistogram name##_;
+  MEGABYTE_HISTOGRAM_RANGE_LIST(HR)
+#undef HR
+
 #define HT(name, caption, max, res) HistogramTimer name##_;
   HISTOGRAM_TIMER_LIST(HT)
 #undef HT