Revert of Remove MessageLoop destruction observer from mojo::Watcher.

ipc/ipc_sync_channel.cc

Index: ipc/ipc_sync_channel.cc
diff --git a/ipc/ipc_sync_channel.cc b/ipc/ipc_sync_channel.cc
index 979c6d3b0883c7e0eb22da2cac2fe78eed3e1eed..e2c6cdba9058b4c9de292105801a4d144bffb13d 100644
--- a/ipc/ipc_sync_channel.cc
+++ b/ipc/ipc_sync_channel.cc
@@ -45,8 +45,9 @@
 // is only used in cases where failure should be impossible) and runs
 // |callback|.
 void RunOnHandleReady(const base::Closure& callback, MojoResult result) {
-  DCHECK_EQ(result, MOJO_RESULT_OK);
-  callback.Run();
+  DCHECK(result == MOJO_RESULT_OK || result == MOJO_RESULT_ABORTED);
+  if (result == MOJO_RESULT_OK)
+    callback.Run();
 }
 
 class PumpMessagesEvent {
@@ -678,9 +679,11 @@
 }
 
 void SyncChannel::OnDispatchHandleReady(MojoResult result) {
-  DCHECK_EQ(result, MOJO_RESULT_OK);
-  sync_context()->GetDispatchEvent()->Reset();
-  sync_context()->DispatchMessages();
+  DCHECK(result == MOJO_RESULT_OK || result == MOJO_RESULT_ABORTED);
+  if (result == MOJO_RESULT_OK) {
+    sync_context()->GetDispatchEvent()->Reset();
+    sync_context()->DispatchMessages();
+  }
 }
 
 void SyncChannel::StartWatching() {