- entries preview available even if debugger agent is disabled,
- less deprecated mirrors usage in debugger-script.js
- no usage of debugger context - zero probability of leaking it.
- better test coverage.

BUG=v8:5510
R=yangguo@chromium.org,jgruber@chromium.org,alph@chromium.org,luoe@chromium.org

Review-Url: https://codereview.chromium.org/2672213002
Cr-Commit-Position: refs/heads/master@{#42978}