Make USB a ContextLifecycleObserver.

third_party/WebKit/Source/modules/webusb/USB.cpp

Index: third_party/WebKit/Source/modules/webusb/USB.cpp
diff --git a/third_party/WebKit/Source/modules/webusb/USB.cpp b/third_party/WebKit/Source/modules/webusb/USB.cpp
index 73ac25da38b5a6fc8ed63b0bede5732fda92f326..3bd8b8fc2ed3e2bf0c951120c0da1548f8850d94 100644
--- a/third_party/WebKit/Source/modules/webusb/USB.cpp
+++ b/third_party/WebKit/Source/modules/webusb/USB.cpp
@@ -75,7 +75,7 @@ public:
 } // namespace
 
 USB::USB(LocalFrame& frame)
-    : LocalFrameLifecycleObserver(&frame)
+    : ContextLifecycleObserver(frame.document())
     , m_client(USBController::from(frame).client())

[haraken, 2016/04/06 00:15:12]

It looks strange that USB is observing ExecutionContext while USBController is
associated with Frame. Would you explain the relationship between Page (which
doesn't change over navigation), Frame (which changes over navigation),
ExecutionContext (which is actually a document in the frame) and USB things?

[Reilly Grant (use Gerrit), 2016/04/06 00:54:34]

The Frame is what provides a connection to Mojo services in Chrome. Currently
that's mediated through the WebUSBClient provided by the USBController but very
soon this constructor will simply request an instance of the
device::usb::DeviceManager Mojo service from frame.serviceRegistry().

I believe that the USB objects in Blink, however, should be owned by a
particular ExecutionContext. I don't want to depend on GC to clean them up
because if the page is refreshed I want old device connections to be cleaned up
immediately so that the page can reconnect as soon as it loads.

 {
     if (m_client)
@@ -128,7 +128,7 @@ ScriptPromise USB::requestDevice(ScriptState* scriptState, const USBDeviceReques
 
 ExecutionContext* USB::getExecutionContext() const
 {
-    return frame() ? frame()->document() : nullptr;
+    return ContextLifecycleObserver::getExecutionContext();
 }
 
 const AtomicString& USB::interfaceName() const
@@ -136,7 +136,7 @@ const AtomicString& USB::interfaceName() const
     return EventTargetNames::USB;
 }
 
-void USB::willDetachFrameHost()
+void USB::contextDestroyed()
 {
     if (m_client)
         m_client->removeObserver(this);
@@ -156,7 +156,7 @@ void USB::onDeviceDisconnected(WebPassOwnPtr<WebUSBDevice> device)
 DEFINE_TRACE(USB)
 {
     RefCountedGarbageCollectedEventTargetWithInlineData<USB>::trace(visitor);
-    LocalFrameLifecycleObserver::trace(visitor);
+    ContextLifecycleObserver::trace(visitor);
 }
 
 } // namespace blink