[wasm] Remove unneeded code path

We are always passing a valid buffer to WasmMemoryObject::New, hence
make it a Handle instead of MaybeHandle, and remove the code to allocate
an empty buffer if none was passed.

R=ahaas@chromium.org

Bug: v8:10949
Change-Id: Ic10dfda741773a8f3d3895f81a83048cfe0eb293
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3702805
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81172}

src/wasm/module-instantiate.cc

@@ -512,9 +512,8 @@ MaybeHandle<WasmInstanceObject> InstanceBuilder::Build() {
     auto maximum_pages =
         static_cast<int>(RoundUp(buffer->byte_length(), wasm::kWasmPageSize) /
                          wasm::kWasmPageSize);
-    memory_object_ =
-        WasmMemoryObject::New(isolate_, memory_buffer_, maximum_pages)
-            .ToHandleChecked();
+    memory_object_ = WasmMemoryObject::New(isolate_, buffer, maximum_pages)
+                         .ToHandleChecked();
   } else {
     // Actual wasm module must have either imported or created memory.
     CHECK(memory_buffer_.is_null());

src/wasm/wasm-objects.cc

@@ -816,16 +816,7 @@ void SetInstanceMemory(Handle<WasmInstanceObject> instance,
 }  // namespace
 
 MaybeHandle<WasmMemoryObject> WasmMemoryObject::New(
-    Isolate* isolate, MaybeHandle<JSArrayBuffer> maybe_buffer, int maximum) {
-  Handle<JSArrayBuffer> buffer;
-  if (!maybe_buffer.ToHandle(&buffer)) {
-    // If no buffer was provided, create a zero-length one.
-    auto backing_store =
-        BackingStore::AllocateWasmMemory(isolate, 0, 0, SharedFlag::kNotShared);
-    if (!backing_store) return {};
-    buffer = isolate->factory()->NewJSArrayBuffer(std::move(backing_store));
-  }
-
+    Isolate* isolate, Handle<JSArrayBuffer> buffer, int maximum) {
   Handle<JSFunction> memory_ctor(
       isolate->native_context()->wasm_memory_constructor(), isolate);
 

src/wasm/wasm-objects.h

@@ -256,7 +256,7 @@ class WasmMemoryObject
   inline bool has_maximum_pages();
 
   V8_EXPORT_PRIVATE static MaybeHandle<WasmMemoryObject> New(
-      Isolate* isolate, MaybeHandle<JSArrayBuffer> buffer, int maximum);
+      Isolate* isolate, Handle<JSArrayBuffer> buffer, int maximum);
 
   V8_EXPORT_PRIVATE static MaybeHandle<WasmMemoryObject> New(Isolate* isolate,
                                                              int initial,