Add "sandboxed" option to "webPreferences".

When "sandboxed" is passed as a web preference for `BrowserWindow`, the newly
created renderer won't run any node.js code/integration, only communicating with
the system via the IPC API of the content module. This is a requirement for
running the renderer under chrome OS-level sandbox.

Beyond that, certain behaviors of AtomBrowserClient are modified when dealing
with sandboxed renderers:

- `OverrideSiteInstanceNavigation` no longer create a new `SiteInstance` for
  every navigation. Instead, it reuses the source `SiteInstance` when not
  navigating to a different site.
- `CanCreateWindow` will return true and allow javascript access.

atom/browser/web_contents_preferences.h

@@ -36,6 +36,8 @@ class WebContentsPreferences
   static void AppendExtraCommandLineSwitches(
       content::WebContents* web_contents, base::CommandLine* command_line);
 
+  static bool IsSandboxed(content::WebContents* web_contents);
+
   // Modify the WebPreferences according to |web_contents|'s preferences.
   static void OverrideWebkitPrefs(
       content::WebContents* web_contents, content::WebPreferences* prefs);