Remove single observer assumption in WebUSBClient.

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

Index: third_party/WebKit/Source/modules/webusb/USB.h
diff --git a/third_party/WebKit/Source/modules/webusb/USB.h b/third_party/WebKit/Source/modules/webusb/USB.h
index fbad005d53dfa6c20683f1b6bb3be61689462d20..065936aece2ba043987e4bfa1c0d4c8356033348 100644
--- a/third_party/WebKit/Source/modules/webusb/USB.h
+++ b/third_party/WebKit/Source/modules/webusb/USB.h
@@ -10,6 +10,7 @@
 #include "core/events/EventTarget.h"
 #include "core/frame/LocalFrameLifecycleObserver.h"
 #include "platform/heap/Handle.h"
+#include "platform/heap/ThreadState.h"
 #include "public/platform/modules/webusb/WebUSBClient.h"
 
 namespace blink {
@@ -26,6 +27,7 @@ class USB final
     DEFINE_WRAPPERTYPEINFO();
     REFCOUNTED_GARBAGE_COLLECTED_EVENT_TARGET(USB);
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(USB);
+    WILL_BE_USING_PRE_FINALIZER(USB, dispose);
 public:
     static USB* create(LocalFrame& frame)
     {
@@ -34,6 +36,8 @@ public:
 
     ~USB() override;
 
+    void dispose();
+
     // USB.idl
     ScriptPromise getDevices(ScriptState*);
     ScriptPromise requestDevice(ScriptState*, const USBDeviceRequestOptions&);