Use TaskScheduler instead of WorkerPool in v8_platform.cc.

gin/test/v8_test.h

Index: gin/test/v8_test.h
diff --git a/gin/test/v8_test.h b/gin/test/v8_test.h
index 635281b16f028603ec5918663d775f5e019f13d7..da5fb6a5d7aa36fb987c9d8ce1f69f7036674c3c 100644
--- a/gin/test/v8_test.h
+++ b/gin/test/v8_test.h
@@ -10,6 +10,7 @@
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
+#include "base/test/scoped_async_task_scheduler.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "v8/include/v8.h"
 
@@ -29,6 +30,11 @@ class V8Test : public testing::Test {
 
  protected:
   base::MessageLoop message_loop_;
+
+  // Required by gin::V8Platform::CallOnBackgroundThread(). Can't be a
+  // ScopedTaskScheduler because v8 synchronously waits for tasks to run.
+  base::test::ScopedAsyncTaskScheduler scoped_async_task_scheduler_;
+
   std::unique_ptr<IsolateHolder> instance_;
   v8::Persistent<v8::Context> context_;