[gin] Do not create a new object on every background task posted from v8.

Bug: 817330
Change-Id: Ifb402ed997f4c2b4d96e1b4124ee518b8700f69a
Reviewed-on: https://chromium-review.googlesource.com/940140Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539823}

gin/v8_platform.cc

@@ -327,7 +327,9 @@ std::shared_ptr<v8::TaskRunner> V8Platform::GetForegroundTaskRunner(
 
 std::shared_ptr<v8::TaskRunner> V8Platform::GetBackgroundTaskRunner(
     v8::Isolate* isolate) {
-  return std::make_shared<V8BackgroundTaskRunner>();
+  static std::shared_ptr<v8::TaskRunner> v8_background_task_runner =
+      std::make_shared<V8BackgroundTaskRunner>();
+  return v8_background_task_runner;
 }
 
 size_t V8Platform::NumberOfAvailableBackgroundThreads() {