clock: Integrate now.go and now_js.go

The monotonic timer already uses performance.now, then we don't
have to have the original code.

internal/clock/now_js.go

@@ -1,35 +0,0 @@
-// Copyright 2016 Hajime Hoshi
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// +build js
-
-package clock
-
-import (
-	"syscall/js"
-	"time"
-)
-
-var (
-	jsPerformance = js.Global().Get("performance")
-	jsNow         = jsPerformance.Get("now").Call("bind", jsPerformance)
-)
-
-func now() int64 {
-	// time.Now() is not reliable until GopherJS supports performance.now().
-	//
-	// performance.now is monotonic:
-	// https://www.w3.org/TR/hr-time-2/#sec-monotonic-clock
-	return int64(jsNow.Invoke().Float() * float64(time.Millisecond))
-}

internal/clock/now_notwindows.go

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build !js
 // +build !windows
 
 package clock