Update GPUBuffer mapping to use v8::BackingStore::EmptyDeleter

Mapped GPUBuffers' memory is owned and externally managed by dawn_wire.
Using EmptyDeleter tells the backing store this allocation does not
hold memory and will not increase GC pressure.

Bug: 1061960
Change-Id: Ia44a5dc1be89d7b55540247c90bce614172fa2ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380086
Commit-Queue: Austin Eng <enga@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802866}

third_party/blink/renderer/modules/webgpu/gpu_buffer.cc

@@ -303,10 +303,7 @@ DOMArrayBuffer* GPUBuffer::CreateArrayBufferForMappedData(void* data,
   DCHECK_LE(data_length, kLargestMappableSize);
 
   ArrayBufferContents contents(data, data_length,
-                               [](void* data, size_t length, void* info) {
-                                 // DataDeleter does nothing because Dawn wire
-                                 // owns the memory.
-                               });
+                               v8::BackingStore::EmptyDeleter);
 
   DOMArrayBuffer* array_buffer = DOMArrayBuffer::Create(contents);
   mapped_array_buffers_.push_back(array_buffer);