[MessagePort] Add usecounter for transferring a closed port.

There is discussion in https://github.com/whatwg/html/pull/3584 about no
longer allowing this, so measure how often this actually is used.

Bug: None
Change-Id: Id4ccf910993df37fa1754954f4fe9fc05cbd4493
Reviewed-on: https://chromium-review.googlesource.com/973693Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544778}

third_party/WebKit/Source/core/messaging/MessagePort.cpp

@@ -167,6 +167,7 @@ Vector<MessagePortChannel> MessagePort::DisentanglePorts(
     return Vector<MessagePortChannel>();
 
   HeapHashSet<Member<MessagePort>> visited;
+  bool has_closed_ports = false;
 
   // Walk the incoming array - if there are any duplicate ports, or null ports
   // or cloned ports, throw an error (per section 8.3.3 of the HTML5 spec).
@@ -185,10 +186,14 @@ Vector<MessagePortChannel> MessagePort::DisentanglePorts(
           "Port at index " + String::Number(i) + " is " + type + ".");
       return Vector<MessagePortChannel>();
     }
+    if (port->closed_)
+      has_closed_ports = true;
     visited.insert(port);
   }
 
   UseCounter::Count(context, WebFeature::kMessagePortsTransferred);
+  if (has_closed_ports)
+    UseCounter::Count(context, WebFeature::kMessagePortTransferClosedPort);
 
   // Passed-in ports passed validity checks, so we can disentangle them.
   Vector<MessagePortChannel> channels;

third_party/WebKit/public/platform/web_feature.mojom

@@ -1881,6 +1881,7 @@ enum WebFeature {
   kDistrustedLegacySymantecSubresource = 2389,
   kVRDisplayGetFrameData = 2390,
   kXMLHttpRequestResponseXML = 2391,
+  kMessagePortTransferClosedPort = 2392,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

tools/metrics/histograms/enums.xml

@@ -17974,6 +17974,7 @@ Called by update_net_error_codes.py.-->
   <int value="2389" label="DistrustedLegacySymantecSubresource"/>
   <int value="2390" label="VRDisplayGetFrameData"/>
   <int value="2391" label="XMLHttpRequestResponseXML"/>
+  <int value="2392" label="MessagePortTransferClosedPort"/>
 </enum>
 
 <enum name="FeedbackSource">