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

content/browser/mojo/mojo_application_host.cc

Index: content/browser/mojo/mojo_application_host.cc
diff --git a/content/browser/mojo/mojo_application_host.cc b/content/browser/mojo/mojo_application_host.cc
index 2491252891316e369215592f97d9b3debeba9c2d..3af797499e0b809e1652411e8670ab62f14ac26e 100644
--- a/content/browser/mojo/mojo_application_host.cc
+++ b/content/browser/mojo/mojo_application_host.cc
@@ -59,4 +59,8 @@ bool MojoApplicationHost::Activate(IPC::Sender* sender,
   return did_activate_;
 }
 
+void MojoApplicationHost::WillDestroySoon() {
+  channel_init_.WillDestroySoon();
+}
+
 }  // namespace content