Mojo: Add the ability to notify a Mojo Channel that it's going to be destroyed soon.

mojo/embedder/embedder.cc

Index: mojo/embedder/embedder.cc
diff --git a/mojo/embedder/embedder.cc b/mojo/embedder/embedder.cc
index 674e7c205073565d53748956f5321622f85ecffe..a9f70c5ef09cc70dd270baccccf1f407a9ccf5ee 100644
--- a/mojo/embedder/embedder.cc
+++ b/mojo/embedder/embedder.cc
@@ -173,10 +173,21 @@ void DestroyChannel(ChannelInfo* channel_info) {
   DCHECK(channel_info);
   DCHECK(channel_info->io_thread_task_runner);
 
+  if (!channel_info->channel) {
+    // Presumably, |Init()| on the channel failed.
+    return;
+  }
+
+  channel_info->channel->WillShutdownSoon();
   channel_info->io_thread_task_runner->PostTask(
       FROM_HERE, base::Bind(&DestroyChannelOnIOThread, channel_info));
 }
 
+void WillDestroyChannelSoon(ChannelInfo* channel_info) {
+  DCHECK(channel_info);
+  channel_info->channel->WillShutdownSoon();
+}
+
 MojoResult CreatePlatformHandleWrapper(
     ScopedPlatformHandle platform_handle,
     MojoHandle* platform_handle_wrapper_handle) {