Add WebUSB layout tests for device disconnection.

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

Index: third_party/WebKit/LayoutTests/usb/resources/usb-helpers.js
diff --git a/third_party/WebKit/LayoutTests/usb/resources/usb-helpers.js b/third_party/WebKit/LayoutTests/usb/resources/usb-helpers.js
index 4ea82a82f85afdf09ba9efc55d07332c696157d0..8d1a1b12eb300a1d4b030edc1465ace044d45d64 100644
--- a/third_party/WebKit/LayoutTests/usb/resources/usb-helpers.js
+++ b/third_party/WebKit/LayoutTests/usb/resources/usb-helpers.js
@@ -371,7 +371,12 @@ function usbMocks(mojo) {
 
       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);
         }