Adds a flag to support "Virtual Time" to the blink scheduler

components/scheduler/child/webthread_impl_for_worker_scheduler.cc

Index: components/scheduler/child/webthread_impl_for_worker_scheduler.cc
diff --git a/components/scheduler/child/webthread_impl_for_worker_scheduler.cc b/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
index 0706d53efdf000187c76eaa80e1460b778b2c3bc..f9ed74319b9d86759fd02dc537fc1e30b92ba492 100644
--- a/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
+++ b/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
@@ -8,6 +8,7 @@
 #include "base/location.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/waitable_event.h"
+#include "base/time/default_tick_clock.h"
 #include "components/scheduler/base/task_queue.h"
 #include "components/scheduler/child/scheduler_task_runner_delegate_impl.h"
 #include "components/scheduler/child/web_scheduler_impl.h"
@@ -43,8 +44,10 @@ WebThreadImplForWorkerScheduler::~WebThreadImplForWorkerScheduler() {
 
 void WebThreadImplForWorkerScheduler::InitOnThread(
     base::WaitableEvent* completion) {
-  task_runner_delegate_ =
-      SchedulerTaskRunnerDelegateImpl::Create(thread_->message_loop());
+  // TODO(alexclarke): Do we need to unify virtual time for workers and the
+  // main thread?

[Sami, 2015/10/30 11:11:20]

One issue here is that workers will see virtual time in Date.now() etc. but
their timers will fire according to wall clock time. I don't think we need to
fix that in this patch but something to be aware of.

[alex clarke (OOO till 29th), 2015/10/30 12:12:25]

Acknowledged.

+  task_runner_delegate_ = SchedulerTaskRunnerDelegateImpl::Create(
+      thread_->message_loop(), make_scoped_ptr(new base::DefaultTickClock()));
   worker_scheduler_ = WorkerScheduler::Create(task_runner_delegate_);
   worker_scheduler_->Init();
   task_runner_ = worker_scheduler_->DefaultTaskRunner();