Expose --enable-sandbox command-line switch.

When `--enable-sandbox` is passed, electron will use chromium sandbox to spawn
all renderers, and every new BrowserWindow will automatically have "sandboxed"
passed as a web preference(since the renderer would not work properly
otherwise).

atom/browser/atom_browser_client.cc

@@ -232,6 +232,7 @@ void AtomBrowserClient::AppendExtraCommandLineSwitches(
   // Copy following switches to child process.
   static const char* const kCommonSwitchNames[] = {
     switches::kStandardSchemes,
+    switches::kEnableSandbox
   };
   command_line->CopySwitchesFrom(
       *base::CommandLine::ForCurrentProcess(),