chore: pass base::StringPiece args by value (#19432)

https://cs.chromium.org/chromium/src/base/strings/string_piece.h?l=14
discusses this, saying "Prefer passing StringPieces by value" because
"[p]assing by value generates slightly smaller code."

shell/common/gin_util.h

@@ -12,7 +12,7 @@ namespace gin_util {
 
 template <typename T>
 bool SetMethod(v8::Local<v8::Object> recv,
-               const base::StringPiece& key,
+               base::StringPiece key,
                const T& callback) {
   v8::Isolate* isolate = v8::Isolate::GetCurrent();
   auto context = isolate->GetCurrentContext();