[CrOS MultiDevice] Make PendingConnectionRequest::GetRequestId() const.

Bug: 824568, 752273
Change-Id: Ie4aac00d5b077c3d1af3114e73447dd52065da75
Reviewed-on: https://chromium-review.googlesource.com/1056213Reviewed-by: James Hawkins <jhawkins@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558050}

chromeos/services/secure_channel/pending_connection_request.h

@@ -29,7 +29,7 @@ class PendingConnectionRequest {
   virtual void HandleConnectionFailure(FailureDetailType failure_detail) = 0;
 
   // Note: Request ID is guaranteed to be unique among all requests.
-  const std::string& GetRequestId() {
+  const std::string& GetRequestId() const {
     static const std::string kRequestId = base::GenerateGUID();
     return kRequestId;
   }