[Jobs API]: Implement new version of GetMaxConcurrency in V8PlatformTest.

It's necessary to temporarily implement both old and new versions
of GetMaxConcurrency() until v8-platform makes the old version optional.

Bug: 1114823
Change-Id: I284f97a36ffb8718c3b42c05a949cfc29f4f1678
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2375505Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801793}

gin/v8_platform_unittest.cc

@@ -74,7 +74,12 @@ TEST(V8PlatformTest, PostJobSimple) {
     explicit Task(std::atomic_size_t* num_tasks_to_run)
         : num_tasks_to_run(num_tasks_to_run) {}
     void Run(v8::JobDelegate* delegate) override { --(*num_tasks_to_run); }
+
+    // TODO(1114823): Cleanup the old version once migration is complete.
     size_t GetMaxConcurrency() const override { return *num_tasks_to_run; }
+    size_t GetMaxConcurrency(size_t /* worker_count*/) const override {
+      return GetMaxConcurrency();
+    }
 
     std::atomic_size_t* num_tasks_to_run;
   };