Bug 1516240 - Part 1: Expose whether a cross-process channel is in use, r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D19177

--HG--
extra : moz-landing-system : lando

ipc/glue/MessageChannel.h

@@ -312,6 +312,11 @@ class MessageChannel : HasResultCodes, MessageLoop::DestructionObserver {
     sIsPumpingMessages = aIsPumping;
   }
 
+  /**
+   * Does this MessageChannel cross process boundaries?
+   */
+  bool IsCrossProcess() const { return mIsCrossProcess; }
+
 #ifdef OS_WIN
   struct MOZ_STACK_CLASS SyncStackFrame {
     SyncStackFrame(MessageChannel* channel, bool interrupt);