[wasm][serialization] Parallelize copy and relocate

The copy&relocate work is fully parallelizable. Even though it's not the
bottleneck, there is no reason to restrict it to a single thread.

R=thibaudm@chromium.org

Bug: v8:11164
Change-Id: I7d0fea66b2a14e36cb60438cff84e4722b7a7993
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2644949
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72366}

src/wasm/wasm-serialization.cc

@@ -576,11 +576,8 @@ class CopyAndRelocTask : public JobTask {
     } while (!delegate->ShouldYield());
   }
 
-  size_t GetMaxConcurrency(size_t worker_count) const override {
-    // Run the CopyAndRelocTask in a single thread for now. We can later bump
-    // this to see if it improves performance.
-    if (worker_count > 0) return 0;
-    return std::min(size_t{1}, from_queue_->NumBatches());
+  size_t GetMaxConcurrency(size_t /* worker_count */) const override {
+    return from_queue_->NumBatches();
   }
 
  private: