* 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/atom_renderer_client.cc
#include "atom/renderer/atom_renderer_client.h"
#include <string>
#include <vector>
#include "atom/common/api/atom_bindings.h"
#include "atom/common/api/event_emitter_caller.h"
#include "atom/common/asar/asar_util.h"
#include "atom/common/atom_constants.h"
#include "atom/common/node_bindings.h"
#include "atom/common/options_switches.h"
#include "atom/renderer/api/atom_api_renderer_ipc.h"
#include "atom/renderer/atom_render_frame_observer.h"
#include "atom/renderer/atom_render_view_observer.h"
#include "atom/renderer/web_worker_observer.h"
#include "base/command_line.h"
#include "content/public/renderer/render_frame.h"
#include "native_mate/dictionary.h"
#include "third_party/WebKit/public/web/WebDocument.h"
#include "third_party/WebKit/public/web/WebLocalFrame.h"
#include "atom/common/node_includes.h"
#include "atom_natives.h"  
namespace atom {
namespace {
bool IsDevToolsExtension(content::RenderFrame* render_frame) {