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/common/options_switches.cc

@@ -117,6 +117,9 @@ const char kDisableBlinkFeatures[] = "disableBlinkFeatures";
 
 namespace switches {
 
+// Enable chromium sandbox.
+const char kEnableSandbox[] = "enable-sandbox";
+
 // Enable plugins.
 const char kEnablePlugins[] = "enable-plugins";
 

atom/common/options_switches.h

@@ -67,6 +67,7 @@ extern const char kDisableBlinkFeatures[];
 
 namespace switches {
 
+extern const char kEnableSandbox[];
 extern const char kEnablePlugins[];
 extern const char kPpapiFlashPath[];
 extern const char kPpapiFlashVersion[];