Mojo: Avoid inappropriate DCHECK when sending invalid handles

It should be safe to send invalid Windows HANDLE values over Mojo, as
we apparently have code relying on this already. Make sure we don't
hit a DCHECK in that case.

TBR=jcivelli@chromium.org

Bug: 859063
Change-Id: I8c9c0c3a31a5904b28a75b78ba74c22440afd83b
Reviewed-on: https://chromium-review.googlesource.com/1120430Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571493}

mojo/edk/system/channel_win.cc

@@ -59,8 +59,10 @@ class ChannelWin : public Channel,
       // If we know the remote process handle, we transfer all outgoing handles
       // to the process now rewriting them in the message.
       std::vector<PlatformHandleInTransit> handles = message->TakeHandles();
-      for (auto& handle : handles)
-        handle.TransferToProcess(remote_process().Clone());
+      for (auto& handle : handles) {
+        if (handle.handle().is_valid())
+          handle.TransferToProcess(remote_process().Clone());
+      }
       message->SetHandles(std::move(handles));
     }