DevTools: [eslint] validate indentation

      This patch switches on eslint validation of indentation in the codebase.

Some files intentionally break indentation -e.g. Tests.js and devtools.js.
These files are marked with &quoteslint-disable indent&quot comment in the
beginning.

This change also found a classic bug in ElementsTreeElement.js

BUG=none

Review-Url: https://codereview.chromium.org/2017813003
Cr-Commit-Position: refs/heads/master@{#396578}