Add V8 shared array allocation sizes histogram.

Add histogram to track sizes of share array buffers in Wasm/Js.

Associated with Chrome CL: https://codereview.chromium.org/2898953003

BUG=chromium:725309

Review-Url: https://codereview.chromium.org/2902773002
Cr-Commit-Position: refs/heads/master@{#45577}

src/counters.h

@@ -977,7 +977,9 @@ class RuntimeCallTimerScope {
      100000, 51)                                                              \
   HR(array_buffer_big_allocations, V8.ArrayBufferLargeAllocations, 0, 4096,   \
      13)                                                                      \
-  HR(array_buffer_new_size_failures, V8.ArrayBufferNewSizeFailures, 0, 4096, 13)
+  HR(array_buffer_new_size_failures, V8.ArrayBufferNewSizeFailures, 0, 4096,  \
+     13)                                                                      \
+  HR(shared_array_allocations, V8.SharedArrayAllocationSizes, 0, 4096, 13)
 
 #define HISTOGRAM_TIMER_LIST(HT)                                               \
   /* Garbage collection timers. */                                             \

src/objects.cc

@@ -19575,6 +19575,9 @@ 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 {