[Slow Reports] Fix ScopedBlockingCall for waitable_event_mac.mm

This is a follow-up to https://chromium-review.googlesource.com/c/1305891

Missed that Mac had its own impl.
Example problematic trace because of this http://crash/18c6999290e01f89

Made all impls match waitable_event_win.cc
(also ignoring the ScopedEventWaitActivity when idle)

R=fdoray@chromium.org

Bug: 899897
Change-Id: Ib5aa1cec487f80a6f1d603658279666ee0d1de60
Reviewed-on: https://chromium-review.googlesource.com/c/1336073Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608511}

base/synchronization/waitable_event_mac.cc

@@ -14,6 +14,7 @@
 #include "base/mac/mac_util.h"
 #include "base/mac/mach_logging.h"
 #include "base/mac/scoped_dispatch_object.h"
+#include "base/optional.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/threading/scoped_blocking_call.h"
 #include "base/threading/thread_restrictions.h"
@@ -111,10 +112,16 @@ bool WaitableEvent::TimedWait(const TimeDelta& wait_delta) {
 }
 
 bool WaitableEvent::TimedWaitUntil(const TimeTicks& end_time) {
-  internal::ScopedBlockingCallWithBaseSyncPrimitives scoped_blocking_call(
-      BlockingType::MAY_BLOCK);
-  // Record the event that this thread is blocking upon (for hang diagnosis).
-  debug::ScopedEventWaitActivity event_activity(this);
+  // Record the event that this thread is blocking upon (for hang diagnosis) and
+  // consider blocked for scheduling purposes. Ignore this for non-blocking
+  // WaitableEvents.
+  Optional<debug::ScopedEventWaitActivity> event_activity;
+  Optional<internal::ScopedBlockingCallWithBaseSyncPrimitives>
+      scoped_blocking_call;
+  if (waiting_is_blocking_) {
+    event_activity.emplace(this);
+    scoped_blocking_call.emplace(BlockingType::MAY_BLOCK);
+  }
 
   mach_msg_empty_rcv_t msg{};
   msg.header.msgh_local_port = receive_right_->Name();

base/synchronization/waitable_event_posix.cc

@@ -163,13 +163,16 @@ bool WaitableEvent::TimedWait(const TimeDelta& wait_delta) {
 }
 
 bool WaitableEvent::TimedWaitUntil(const TimeTicks& end_time) {
+  // Record the event that this thread is blocking upon (for hang diagnosis) and
+  // consider it blocked for scheduling purposes. Ignore this for non-blocking
+  // WaitableEvents.
+  Optional<debug::ScopedEventWaitActivity> event_activity;
   Optional<internal::ScopedBlockingCallWithBaseSyncPrimitives>
       scoped_blocking_call;
-  if (waiting_is_blocking_)
+  if (waiting_is_blocking_) {
+    event_activity.emplace(this);
     scoped_blocking_call.emplace(BlockingType::MAY_BLOCK);
-
-  // Record the event that this thread is blocking upon (for hang diagnosis).
-  debug::ScopedEventWaitActivity event_activity(this);
+  }
 
   const bool finite_time = !end_time.is_max();
 

base/synchronization/waitable_event_win.cc

@@ -55,7 +55,7 @@ bool WaitableEvent::IsSignaled() {
 
 void WaitableEvent::Wait() {
   // Record the event that this thread is blocking upon (for hang diagnosis) and
-  // consider blocked for scheduling purposes. Ignore this for non-blocking
+  // consider it blocked for scheduling purposes. Ignore this for non-blocking
   // WaitableEvents.
   Optional<debug::ScopedEventWaitActivity> event_activity;
   Optional<internal::ScopedBlockingCallWithBaseSyncPrimitives>
@@ -113,7 +113,7 @@ bool WaitableEvent::TimedWait(const TimeDelta& wait_delta) {
     return IsSignaled();
 
   // Record the event that this thread is blocking upon (for hang diagnosis) and
-  // consider blocked for scheduling purposes. Ignore this for non-blocking
+  // consider it blocked for scheduling purposes. Ignore this for non-blocking
   // WaitableEvents.
   Optional<debug::ScopedEventWaitActivity> event_activity;
   Optional<internal::ScopedBlockingCallWithBaseSyncPrimitives>
@@ -134,7 +134,7 @@ bool WaitableEvent::TimedWaitUntil(const TimeTicks& end_time) {
     return IsSignaled();
 
   // Record the event that this thread is blocking upon (for hang diagnosis) and
-  // consider blocked for scheduling purposes. Ignore this for non-blocking
+  // consider it blocked for scheduling purposes. Ignore this for non-blocking
   // WaitableEvents.
   Optional<debug::ScopedEventWaitActivity> event_activity;
   Optional<internal::ScopedBlockingCallWithBaseSyncPrimitives>