Revert Mojo-based SyncChannel waiting again

ipc/ipc_sync_message.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ipc/ipc_sync_message.h"
 
 #include <stdint.h>
 
 #include <stack>
 
 #include "base/atomic_sequence_num.h"
+#include "base/lazy_instance.h"
 #include "base/logging.h"
+#include "base/synchronization/waitable_event.h"
 #include "build/build_config.h"
 
 namespace {
 
+struct WaitableEventLazyInstanceTraits
+    : public base::DefaultLazyInstanceTraits<base::WaitableEvent> {
+  static base::WaitableEvent* New(void* instance) {
+    // Use placement new to initialize our instance in our preallocated space.
+    return new (instance)
+        base::WaitableEvent(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::SIGNALED);
+  }
+};
+
+base::LazyInstance<base::WaitableEvent, WaitableEventLazyInstanceTraits>
+    dummy_event = LAZY_INSTANCE_INITIALIZER;
+
 base::StaticAtomicSequenceNumber g_next_id;
 
 }  // namespace
 
 namespace IPC {
 
 #define kSyncMessageHeaderSize 4
 
 SyncMessage::SyncMessage(int32_t routing_id,
                          uint32_t type,
                          PriorityValue priority,
                          MessageReplyDeserializer* deserializer)
     : Message(routing_id, type, priority),
-      deserializer_(deserializer) {
+      deserializer_(deserializer),
+      pump_messages_event_(NULL) {
   set_sync();
   set_unblock(true);
 
   // Add synchronous message data before the message payload.
   SyncHeader header;
   header.message_id = g_next_id.GetNext();
   WriteSyncHeader(this, header);
 }
 
 SyncMessage::~SyncMessage() {
 }
 
 MessageReplyDeserializer* SyncMessage::GetReplyDeserializer() {
   DCHECK(deserializer_.get());
   return deserializer_.release();
 }
 
+void SyncMessage::EnableMessagePumping() {
+  DCHECK(!pump_messages_event_);
+  set_pump_messages_event(dummy_event.Pointer());
+}
+
 bool SyncMessage::IsMessageReplyTo(const Message& msg, int request_id) {
   if (!msg.is_reply())
     return false;
 
   return GetMessageId(msg) == request_id;
 }
 
 base::PickleIterator SyncMessage::GetDataIterator(const Message* msg) {
   base::PickleIterator iter(*msg);
   if (!iter.SkipBytes(kSyncMessageHeaderSize))
@@ skipping 56 matching lines @@
 
   return true;
 }
 
 
 bool MessageReplyDeserializer::SerializeOutputParameters(const Message& msg) {
   return SerializeOutputParameters(msg, SyncMessage::GetDataIterator(&msg));
 }
 
 }  // namespace IPC