Use TaskScheduler instead of WorkerPool in v8_platform.cc.

mojo/edk/js/tests/js_to_cpp_tests.cc

Index: mojo/edk/js/tests/js_to_cpp_tests.cc
diff --git a/mojo/edk/js/tests/js_to_cpp_tests.cc b/mojo/edk/js/tests/js_to_cpp_tests.cc
index 2e7a39e0b8eaa977461b974c17e9100fba1170a3..530121e58d6af6d6ba8deee93513462d4a618cef 100644
--- a/mojo/edk/js/tests/js_to_cpp_tests.cc
+++ b/mojo/edk/js/tests/js_to_cpp_tests.cc
@@ -15,6 +15,7 @@
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/test/scoped_async_task_scheduler.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "gin/array_buffer.h"
 #include "gin/public/isolate_holder.h"
@@ -419,6 +420,10 @@ class JsToCppTest : public testing::Test {
   base::MessageLoop loop;
   base::RunLoop run_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;
+
   DISALLOW_COPY_AND_ASSIGN(JsToCppTest);
 };