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

client/internal/common/json.go

Index: client/internal/common/json.go
diff --git a/client/internal/common/json.go b/client/internal/common/json.go
deleted file mode 100644
index d1b6c1ac424b99e7fdce3b11d9b2f911b1900e5b..0000000000000000000000000000000000000000
--- a/client/internal/common/json.go
+++ /dev/null
@@ -1,30 +0,0 @@
-// 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
-}