* Output of `node_modules/.bin/electron --version`:
* Operating System (Platform and Version):
* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable): 3.0.4

**Expected Behavior**
devtools extension's page (`manifest.json`'s `devtools_page`) have electron's global `chrome.*` extension api context constructed.

**Actual behavior**
`devtools_page` doesn't have context, any calls to `chrome.*` will fail

**To Reproduce**
```
const {app, BrowserWindow} = require('electron')

//enable mixed sandbox asap
app.enableMixedSandbox();

let mainWindow

function createWindow () {
  mainWindow = new BrowserWindow({width: 1024, height: 768})

 mainWindow.loadURL('https://reactjs.org')
  
  //other devtools extension can be used instead as long as `devtools_page` access chrome.* api
  require('devtron').install();
  mainWindow.webContents.openDevTools()

  mainWindow.on('closed', function () {
    mainWindow = null
  })
}

app.on('ready', createWindow)

app.on('window-all-closed', function () {
  if (process.platform !== 'darwin') {
    app.quit()
  }
})

app.on('activate', function () {
  if (mainWindow === null) {
    createWindow()
  }
})
```

devtron is used for simplified repro, but any other devtools (i.e react devtools) will have same symptoms.
launch electron, then open devtools will emit console logs like
```
[10800:1104/011325.639:ERROR:CONSOLE(7)] "Uncaught ReferenceError: chrome is not defined", source: chrome-extension://devtron/static/devtron.html (7)
```

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Additional Information**
Add any other context about the problem here.

atom/renderer/atom_sandboxed_renderer_client.cc
#include "atom/renderer/atom_sandboxed_renderer_client.h"
#include "atom/common/api/api_messages.h"
#include "atom/common/api/atom_bindings.h"
#include "atom/common/application_info.h"
#include "atom/common/native_mate_converters/string16_converter.h"
#include "atom/common/native_mate_converters/value_converter.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 "base/base_paths.h"
#include "base/command_line.h"
#include "base/files/file_path.h"
#include "base/path_service.h"
#include "base/process/process_handle.h"
#include "content/public/renderer/render_frame.h"
#include "native_mate/dictionary.h"
#include "third_party/blink/public/web/blink.h"
#include "third_party/blink/public/web/web_document.h"
#include "atom/common/node_includes.h"
#include "atom_natives.h"  
namespace atom {
namespace {
const char kIpcKey[] = "ipcNative";
const char kModuleCacheKey[] = "native-module-cache";
bool IsDevTools(content::RenderFrame* render_frame) {