* Electron version: 1.7.3
* Operating system: Windows, MacOS, Linux

### Expected behavior
Context Isolation should work when sandbox option is set to true

### Actual behavior
Context Isolation should work when sandbox option is set to true

### How to reproduce

Turn on contextIsolation and sandbox on browser window. contextIsolation doesn't work. 

contextIsolation work needs to happen on sandbox renderer client as well. 

cc: @tarruda @kevinsawicki @chandrachivukula
atom/renderer/renderer_client_base.cc
#include "atom/renderer/renderer_client_base.h"
#include <string>
#include <vector>
#include "atom/common/atom_constants.h"
#include "atom/common/color_util.h"
#include "atom/common/native_mate_converters/value_converter.h"
#include "atom/common/options_switches.h"
#include "atom/renderer/atom_autofill_agent.h"
#include "atom/renderer/atom_render_frame_observer.h"
#include "atom/renderer/content_settings_observer.h"
#include "atom/renderer/guest_view_container.h"
#include "atom/renderer/preferences_manager.h"
#include "base/command_line.h"
#include "base/strings/string_split.h"
#include "chrome/renderer/media/chrome_key_systems.h"