perf(serde_v8): serde_v8::StringOrBuffer return JS ArrayBuffer inst…

…ead of Uint8Array (#16360)

Towards #16315

ext/websocket/01_websocket.js

@@ -440,7 +440,7 @@
             if (this.binaryType === "blob") {
               data = new Blob([value]);
             } else {
-              data = value.buffer;
+              data = value;
             }
 
             const event = new MessageEvent("message", {

serde_v8/magic/string_or_buffer.rs

@@ -29,7 +29,24 @@ impl ToV8 for StringOrBuffer {
     scope: &mut v8::HandleScope<'a>,
   ) -> Result<v8::Local<'a, v8::Value>, crate::Error> {
     match self {
-      Self::Buffer(buf) => crate::to_v8(scope, buf),
+      Self::Buffer(buf) => {
+        let buf: Box<[u8]> = match buf {
+          ZeroCopyBuf::FromV8(buf) => {
+            let value: &[u8] = buf;
+            value.into()
+          }
+          ZeroCopyBuf::Temp(_) => unreachable!(),
+          ZeroCopyBuf::ToV8(ref mut x) => {
+            x.take().expect("ZeroCopyBuf was empty")
+          }
+        };
+        let backing_store =
+          v8::ArrayBuffer::new_backing_store_from_boxed_slice(buf);
+        Ok(
+          v8::ArrayBuffer::with_backing_store(scope, &backing_store.into())
+            .into(),
+        )
+      }
       Self::String(s) => crate::to_v8(scope, s),
     }
   }