Refactor sandbox preload initialization. (#12877)

Use a single synchronous IPC call to retrieve data required by early
sandbox scripts. This has two purposes:

- Optimize preload script initialization by:
  - Using one synchronous IPC call to retrieve preload script,
  webContentsId (more on that later), process.{platform,execPath,env}
  - Lazy loading as many modules as possible.
- Fix #12316 for sandbox. @MarshallOfSound addressed the issue in
  #12342, but it was still present in sandbox mode. By loading
  webContentsId very early and skipping remote module at early
  startup, we fix it for sandbox.

lib/browser/rpc-server.js

@@ -3,6 +3,7 @@
 const {Buffer} = require('buffer')
 const electron = require('electron')
 const {EventEmitter} = require('events')
+const fs = require('fs')
 const v8Util = process.atomBinding('v8_util')
 
 const {ipcMain, isPromise, webContents} = electron
@@ -448,3 +449,23 @@ ipcMain.on('ELECTRON_BROWSER_WINDOW_CLOSE', function (event) {
   }
   event.returnValue = null
 })
+
+ipcMain.on('ELECTRON_BROWSER_SANDBOX_LOAD', function (event, preloadPath) {
+  let preloadSrc = null
+  let preloadError = null
+  if (preloadPath) {
+    try {
+      preloadSrc = fs.readFileSync(preloadPath).toString()
+    } catch (err) {
+      preloadError = {stack: err ? err.stack : (new Error(`Failed to load "${preloadPath}"`)).stack}
+    }
+  }
+  event.returnValue = {
+    preloadSrc: preloadSrc,
+    preloadError: preloadError,
+    webContentsId: event.sender.getId(),
+    platform: process.platform,
+    execPath: process.execPath,
+    env: process.env
+  }
+})