chore: add a DCHECK to confirm the BigBuffer is full

Co-authored-by: Charles Kerr <charles@charleskerr.com>

shell/common/api/electron_api_clipboard.cc

@@ -114,8 +114,10 @@ void Clipboard::WriteBuffer(const std::string& format,
   CHECK(buffer->IsArrayBufferView());
   v8::Local<v8::ArrayBufferView> buffer_view = buffer.As<v8::ArrayBufferView>();
   const size_t n_bytes = buffer_view->ByteLength();
-  mojo_base::BigBuffer big_buffer(n_bytes);
-  buffer_view->CopyContents(big_buffer.data(), n_bytes);
+  mojo_base::BigBuffer big_buffer{n_bytes};
+  [[maybe_unused]] const size_t n_got =
+      buffer_view->CopyContents(big_buffer.data(), n_bytes);
+  DCHECK_EQ(n_got, n_bytes);
 
   ui::ScopedClipboardWriter writer(GetClipboardBuffer(args));
   writer.WriteUnsafeRawData(base::UTF8ToUTF16(format), std::move(big_buffer));