Add a UseCounter for SharedArrayBuffer creation.

Blink CL: https://chromium-review.googlesource.com/c/chromium/src/+/1944474

Bug: chromium:1029700
Change-Id: I91936942b21d133e06f2583a4e3c70951e5e86f4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1946348
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65290}

include/v8.h

@@ -8248,6 +8248,7 @@ class V8_EXPORT Isolate {
     kCallInDetachedWindowByOtherReason = 88,
     kCallInDetachedWindowByOtherReasonAfter10s = 89,
     kCallInDetachedWindowByOtherReasonAfter1min = 90,
+    kSharedArrayBufferConstructed = 91,
 
     // If you add new values here, you'll also need to update Chromium's:
     // web_feature.mojom, use_counter_callback.cc, and enums.xml. V8 changes to

src/objects/js-array-buffer.cc

@@ -49,6 +49,11 @@ void JSArrayBuffer::Setup(SharedFlag shared,
   } else {
     Attach(std::move(backing_store));
   }
+
+  if (shared == SharedFlag::kShared) {
+    GetIsolate()->CountUsage(
+        v8::Isolate::UseCounterFeature::kSharedArrayBufferConstructed);
+  }
 }
 
 void JSArrayBuffer::Attach(std::shared_ptr<BackingStore> backing_store) {