service worker: Fix EVALUATE_SCRIPT trace slice timing

Before this CL ServiceWorkerContextClient called BEGIN macro
on the main thread but called END macro on worker thread.
Call BEGIN macro on worker thread so that it can have
matching BEGIN/END pair on worker thread.

Bug: 926031
Change-Id: Ic078cae5b7347d4e68e21f088b1c28277fd47f6b
Reviewed-on: https://chromium-review.googlesource.com/c/1441572
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Auto-Submit: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626852}

content/renderer/service_worker/service_worker_context_client.cc

@@ -680,11 +680,11 @@ void ServiceWorkerContextClient::WorkerContextStarted(
 
   TRACE_EVENT_NESTABLE_ASYNC_END0("ServiceWorker", "START_WORKER_CONTEXT",
                                   this);
-  TRACE_EVENT_NESTABLE_ASYNC_BEGIN0("ServiceWorker", "EVALUATE_SCRIPT", this);
 }
 
 void ServiceWorkerContextClient::WillEvaluateScript() {
   DCHECK(worker_task_runner_->RunsTasksInCurrentSequence());
+  TRACE_EVENT_NESTABLE_ASYNC_BEGIN0("ServiceWorker", "EVALUATE_SCRIPT", this);
   start_timing_->script_evaluation_start_time = base::TimeTicks::Now();
 
   // Temporary CHECK for https://crbug.com/881100