Move WriteJSONFile to its sole caller's package, with some refactoring.

client/internal/common/json.go

-// Copyright 2015 The LUCI Authors. All rights reserved.
-// Use of this source code is governed under the Apache License, Version 2.0
-// that can be found in the LICENSE file.
-
-package common
-
-import (
-        "encoding/json"
-        "fmt"
-        "os"
-)
-
-// WriteJSONFile writes object as json encoded into filePath with 2 spaces
-// indentation. File permission is set to user only.
-func WriteJSONFile(filePath string, object interface{}) error {
-        d, err := json.MarshalIndent(object, "", "  ")
-        if err != nil {
-                return fmt.Errorf("failed to encode %s: %s", filePath, err)
-        }
-
-        f, err := os.OpenFile(filePath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
-        if err != nil {
-                return fmt.Errorf("failed to open %s: %s", filePath, err)
-        }
-        defer f.Close()
-        if _, err := f.Write(d); err != nil {
-                return fmt.Errorf("failed to write %s: %s", filePath, err)
-        }
-        return nil
-}