Remove variable shadowing in blink/modules/peerconnection

In an effort to reduce or even ban variable shadowing, this renames
a variable to avoid such shadowing. I'm interested in prohibiting
shadowing because I think it might prevent potential jumbo problems.

The warning this avoids is:
third_party/blink/renderer/modules/peerconnection/rtc_peer_connection.cc:1200:20: error: declaration shadows a local variable [-Werror,-Wshadow]
  for (auto const& transport_iterator : dtls_transports_by_mid_) {
                   ^
third_party/blink/renderer/modules/peerconnection/rtc_peer_connection.cc:1187:8: note: previous declaration is here
  auto transport_iterator = dtls_transports_by_mid_.find(mid);
       ^

Bug: 923510
Change-Id: Ia9e1734c1bd1aa42bf4b13f6d2b43fe28b6ed511
Reviewed-on: https://chromium-review.googlesource.com/c/1478872
Commit-Queue: Daniel Bratell <bratell@opera.com>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Daniel Bratell <bratell@opera.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633746}

third_party/blink/renderer/modules/peerconnection/rtc_peer_connection.cc

@@ -1184,15 +1184,15 @@ RTCDtlsTransport* RTCPeerConnection::LookupDtlsTransportByMid(String mid) {
     return nullptr;
   // Check for previously created RTCDtlsTransport objects referencing
   // this transport.
-  auto transport_iterator = dtls_transports_by_mid_.find(mid);
-  if (transport_iterator != dtls_transports_by_mid_.end()) {
-    if (transport_iterator->value->native_transport() !=
+  auto transport_lookup_result = dtls_transports_by_mid_.find(mid);
+  if (transport_lookup_result != dtls_transports_by_mid_.end()) {
+    if (transport_lookup_result->value->native_transport() !=
         native_transport.get()) {
       // The mid's transport has changed. Erase the reference to
       // the old transport, and continue.
-      dtls_transports_by_mid_.erase(transport_iterator);
+      dtls_transports_by_mid_.erase(transport_lookup_result);
     } else {
-      return transport_iterator->value;
+      return transport_lookup_result->value;
     }
   }