[mojo] Add testing support to BrowserInterfaceBrokerProxy

This change adds a SetBinderForTesting() method to
BrowserInterfaceBrokerProxy and modifies MojoInterfaceInterceptor to
call this new method if a script passes a new "useBrowserInterfaceBroker"
parameter.
Also converts IdleManager to use BrowserInterfaceBrokerProxy as a use
case for overriding interface handlers in web tests.

Bug: 985118
Change-Id: I8dc3fc1c4beb2ae375a2534b03cd890ce1e3b52c

idle-detection/mock.js

@@ -64,7 +64,7 @@ function intercept() {
   let result = new FakeIdleMonitor();
 
   let binding = new mojo.Binding(blink.mojom.IdleManager, result);
-  let interceptor = new MojoInterfaceInterceptor(blink.mojom.IdleManager.name);
+  let interceptor = new MojoInterfaceInterceptor(blink.mojom.IdleManager.name, "context", true);
   interceptor.oninterfacerequest = (e) => {
     binding.bind(e.handle);
   }