Add V8 shared array allocation sizes histogram.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index ce840e228d6851c83bcfaee439584c724718d362..00ecb41242d2f7a4c4326b2cfbf5c431227cfe95 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -19449,6 +19449,9 @@ bool JSArrayBuffer::SetupAllocatingData(Handle<JSArrayBuffer> array_buffer,
     if (allocated_length >= MB)
       isolate->counters()->array_buffer_big_allocations()->AddSample(
           ConvertToMb(allocated_length));
+    if (shared == SharedFlag::kShared)
+      isolate->counters()->shared_array_allocations()->AddSample(
+          ConvertToMb(allocated_length));
     if (initialize) {
       data = isolate->array_buffer_allocator()->Allocate(allocated_length);
     } else {