* 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 "chrome/renderer/pepper/pepper_helper.h"
#include "chrome/renderer/printing/print_web_view_helper.h"
#include "chrome/renderer/tts_dispatcher.h"
#include "content/public/common/content_constants.h"
#include "content/public/renderer/render_view.h"
#include "native_mate/dictionary.h"
#include "third_party/WebKit/public/web/WebCustomElement.h"
#include "third_party/WebKit/public/web/WebFrameWidget.h"
#include "third_party/WebKit/public/web/WebKit.h"
#include "third_party/WebKit/public/web/WebPluginParams.h"
#include "third_party/WebKit/public/web/WebScriptSource.h"
#include "third_party/WebKit/public/web/WebSecurityPolicy.h"
#include "third_party/WebKit/Source/platform/weborigin/SchemeRegistry.h"
#if defined(OS_MACOSX)
#include "base/mac/mac_util.h"
#include "base/strings/sys_string_conversions.h"
#endif
#if defined(OS_WIN)
#include <shlobj.h>
#endif
namespace atom {
namespace {
v8::Local<v8::Value> GetRenderProcessPreferences(
    const PreferencesManager* preferences_manager, v8::Isolate* isolate) {
  if (preferences_manager->preferences())
    return mate::ConvertToV8(isolate, *preferences_manager->preferences());
  else
    return v8::Null(isolate);
}
std::vector<std::string> ParseSchemesCLISwitch(const char* switch_name) {