Add WebUSB layout tests for device disconnection.

third_party/WebKit/LayoutTests/usb/resources/usb-helpers.js

 'use strict';
 
 // TODO(reillyg): Remove when jyasskin upstreams this to testharness.js:
 // https://crbug.com/509058.
 function callWithKeyDown(functionCalledOnKeyPress) {
   return new Promise(resolve => {
     function onKeyPress() {
       document.removeEventListener('keypress', onKeyPress, false);
       resolve(functionCalledOnKeyPress());
     }
@@ skipping 353 matching lines @@
             devices_added: this.addedDevices_,
             devices_removed: this.removedDevices_
           }
         });
         this.addedDevices_ = [];
         this.removedDevices_ = [];
       }
 
       getDevice(guid, pipe) {
         let device = this.mockDevices_.get(guid);
-        if (device !== undefined) {
+        if (device === undefined) {
+          // In C++ this happens automatically if the pipe is not bound to a
+          // new device but must be explicit in JavaScript to avoid dependency
+          // on GC timing.
+          mojo.core.close(pipe);

[Ken Rockot(use gerrit already), 2016/03/02 01:54:19]

nit: I'm not sure this comment belongs here. It's really a documentation bug for
Mojo if it's not clear that people need to close their handles. In general there
is no implicit assumption that C++ APIs look like JS APIs AFAIK.

[Reilly Grant (use Gerrit), 2016/03/02 19:13:26]

Agreed.

+        } else {
           var mock = new MockDevice(device.info, pipe);
           device.handles.push(mock);
         }
       }
     }
 
     class MockPermissionBubble
         extends permissionBubble.WebUsbPermissionBubble.stubClass {
       constructor() {
         super();
@@ skipping 43 matching lines @@
 }
 
 function usb_test(func, name, properties) {
   mojo_test(mojo => usbMocks(mojo).then(usb => {
     let result = Promise.resolve(func(usb));
     let cleanUp = () => usb.mockDeviceManager.reset();
     result.then(cleanUp, cleanUp);
     return result;
   }), name, properties);
 }