* 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
base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  std::string custom_schemes = command_line->GetSwitchValueASCII(switch_name);
  return base::SplitString(
      custom_schemes, ",", base::TRIM_WHITESPACE, base::SPLIT_WANT_NONEMPTY);
}
}  
RendererClientBase::RendererClientBase() {
  
  std::vector<std::string> standard_schemes_list =
      ParseSchemesCLISwitch(switches::kStandardSchemes);
  for (const std::string& scheme : standard_schemes_list)
    url::AddStandardScheme(scheme.c_str(), url::SCHEME_WITHOUT_PORT);
}
RendererClientBase::~RendererClientBase() {
}
void RendererClientBase::AddRenderBindings(
    v8::Isolate* isolate,
    v8::Local<v8::Object> binding_object) {
  mate::Dictionary dict(isolate, binding_object);
  dict.SetMethod(
      "getRenderProcessPreferences",
      base::Bind(GetRenderProcessPreferences, preferences_manager_.get()));
}
void RendererClientBase::RenderThreadStarted() {