[mojo] Allow invitations with null process handle

Some Chrome OS services actually rely on this being OK since they don't
have the PID of the target process available. Presumably they would fail
if DCHECKs were enabled.

This removes an unnecessary DCHECK so that such code can be exercised in
chromium tests.

Bug: None
Change-Id: Id546644aae7222eaf76b9a279f0c99af0764573c
Reviewed-on: https://chromium-review.googlesource.com/1140280Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575692}

mojo/core/channel.h

@@ -276,7 +276,6 @@ class MOJO_SYSTEM_IMPL_EXPORT Channel
   // connected. If called at all, must be called only once, and before Start().
   void set_remote_process(ScopedProcessHandle remote_process) {
     DCHECK(!remote_process_.is_valid());
-    DCHECK(remote_process.is_valid());
     remote_process_ = std::move(remote_process);
   }
   const ScopedProcessHandle& remote_process() const { return remote_process_; }