Bug 1018966 - Part 1: Add the main warn only sandbox machinery - with no Chromium code changes. r=bsmedberg
This change also includes the content sandboxing code on Windows Nightly by defining MOZ_CONTENT_SANDBOX=1.
Whether the content sandbox is disabled, in warn only mode, or enabled is controlled by a new pref: browser.tabs.remote.sandbox=(off/warn/on)
Bug 1018966 - Part 2: Make warn only sandbox changes to the Chromium code. r=tabraldes
Bug 877605 - GTK3 - fixed tab widget rendering. r=karlt