Progress on supporting sandbox in electron started in #6919, but the remaining work will be done by future PRs. This issue will be used to track progress on all relevant tasks.

• Add sandbox option to webPreferences of BrowserWindow. (Add sandbox option and support native window.open #6919)
• Support chromium native window.open API in sandboxed renderers. (Add sandbox option and support native window.open #6919)
• Extract code that is not specific to node.js from AtomRendererClient into a base class that is also shared by AtomSandboxedRendererClient.
• Patch libchromiumcontent to allow reuse of code that determines which navigations should spawn a new renderer. More details here
• Add sandbox option to <webview> tag.
• Expose node.js builtin modules via IPC to sandboxed renderers.
• Reuse most javascript code under lib/renderer/ and lib/common. The goal is to support the full electron API in sandboxed renderers.
• Add documentation