Revise counting of allocated RTCPeerConnections

The old count wouldn't count RTCPeerConnections allocated when returning
an error saying "too many PeerConnections", but those would be garbage
collected later.

This change fixes that issue.

Tested: Before the fix, a different test failed on the (new) DCHECK.
After the fix, the same test didn't fail.

Bug: 827064
Change-Id: I4765ce7043fd8a4e0c9c54dc1ec403e986ffed77
Reviewed-on: https://chromium-review.googlesource.com/1061176Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561044}

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

@@ -543,19 +543,18 @@ RTCPeerConnection::RTCPeerConnection(ExecutionContext* context,
       sdp_semantics_(configuration.sdp_semantics) {
   Document* document = ToDocument(GetExecutionContext());
 
+  InstanceCounters::IncrementCounter(
+      InstanceCounters::kRTCPeerConnectionCounter);
   // If we fail, set |m_closed| and |m_stopped| to true, to avoid hitting the
   // assert in the destructor.
-
   if (InstanceCounters::CounterValue(
-          InstanceCounters::kRTCPeerConnectionCounter) >= kMaxPeerConnections) {
+          InstanceCounters::kRTCPeerConnectionCounter) > kMaxPeerConnections) {
     closed_ = true;
     stopped_ = true;
     exception_state.ThrowDOMException(kUnknownError,
                                       "Cannot create so many PeerConnections");
     return;
   }
-  InstanceCounters::IncrementCounter(
-      InstanceCounters::kRTCPeerConnectionCounter);
   if (!document->GetFrame()) {
     closed_ = true;
     stopped_ = true;
@@ -598,6 +597,8 @@ RTCPeerConnection::~RTCPeerConnection() {
   DCHECK(closed_ || stopped_);
   InstanceCounters::DecrementCounter(
       InstanceCounters::kRTCPeerConnectionCounter);
+  DCHECK(InstanceCounters::CounterValue(
+             InstanceCounters::kRTCPeerConnectionCounter) >= 0);
 }
 
 void RTCPeerConnection::Dispose() {