Fix a bug in UserScriptLoader.

1. Fix a misuse of a released pointer in UserScriptLoader;
2. Add initilaztions for the variables that will be picked in UserScript.

BUG=461052

Review URL: https://codereview.chromium.org/982113002

Cr-Commit-Position: refs/heads/master@{#319329}
3 files changed