ServiceWorker: Make NextEventId() thread-safe

NextEventId(), which is used in ServiceWorkerTimeoutTimer is not thread-safe.
I fixed this problem by using AtomicSequenceNumber.
This ensures event ids dispatched to service workers in a process are unique in the process.

Bug: 869706
Change-Id: Ia19aa180007479387463d3d0725dae052da17632
Reviewed-on: https://chromium-review.googlesource.com/1158092
Commit-Queue: Momoko Hattori <momohatt@google.com>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579749}

content/renderer/service_worker/service_worker_timeout_timer.cc

@@ -4,6 +4,7 @@
 
 #include "content/renderer/service_worker/service_worker_timeout_timer.h"
 
+#include "base/atomic_sequence_num.h"
 #include "base/stl_util.h"
 #include "base/time/default_tick_clock.h"
 #include "base/time/time.h"
@@ -17,9 +18,10 @@ namespace {
 int NextEventId() {
   // Event id should not start from zero since HashMap in Blink requires
   // non-zero keys.
-  static int s_next_event_id = 1;
-  CHECK_LT(s_next_event_id, std::numeric_limits<int>::max());
-  return s_next_event_id++;
+  static base::AtomicSequenceNumber s_event_id_sequence;
+  int next_event_id = s_event_id_sequence.GetNext() + 1;
+  CHECK_LT(next_event_id, std::numeric_limits<int>::max());
+  return next_event_id;
 }
 
 }  // namespace

content/renderer/service_worker/service_worker_timeout_timer.h

@@ -51,8 +51,9 @@ class CONTENT_EXPORT ServiceWorkerTimeoutTimer {
   ~ServiceWorkerTimeoutTimer();
 
   // StartEvent() should be called at the beginning of an event. It returns an
-  // event id. The event id should be passed to EndEvent() when the event has
-  // finished. If there are pending tasks queued by PushPendingTask(), they will
+  // event id, which is unique among threads in the same process.
+  // The event id should be passed to EndEvent() when the event has finished.
+  // If there are pending tasks queued by PushPendingTask(), they will
   // run in order synchronouslly in StartEvent().
   // See the class comment to know when |abort_callback| runs.
   int StartEvent(base::OnceCallback<void(int /* event_id */)> abort_callback);