Remove single observer assumption in WebUSBClient.

third_party/WebKit/public/platform/modules/webusb/WebUSBClient.h

Index: third_party/WebKit/public/platform/modules/webusb/WebUSBClient.h
diff --git a/third_party/WebKit/public/platform/modules/webusb/WebUSBClient.h b/third_party/WebKit/public/platform/modules/webusb/WebUSBClient.h
index aae5827fdd6563de8325798831f3ee8db04b7f95..18c4a5c8d06876778331c31d9e58cd3ec91f84d1 100644
--- a/third_party/WebKit/public/platform/modules/webusb/WebUSBClient.h
+++ b/third_party/WebKit/public/platform/modules/webusb/WebUSBClient.h
@@ -40,8 +40,11 @@ public:
     // Ownership of the WebUSBClientRequestDeviceCallbacks is transferred to the client.
     virtual void requestDevice(const WebUSBDeviceRequestOptions&, WebUSBClientRequestDeviceCallbacks*) = 0;
 
-    // Sets the observer of device changes through the WebUSBClient.
-    virtual void setObserver(Observer*) = 0;
+    // Adds an observer of device changes to the WebUSBClient.
+    virtual void addObserver(Observer*) = 0;
+
+    // Removes an observer of device changes from the WebUSBClient.
+    virtual void removeObserver(Observer*) = 0;
 };
 
 } // namespace blink