With flag we can debug injected-script-source in inspector-test or from DevTools frontend as regular user code. We need this when working on new features or debugging issues, it's for internal purpose only and doesn't provide any benefits for end users.

Flag: --expose-inspector-scripts

BUG=none
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2767873002
Cr-Commit-Position: refs/heads/master@{#44039}