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/renderer/api/remote.js

@@ -306,10 +306,13 @@ exports.getCurrentWebContents = () => {
 
 const CONTEXT_ARG = '--context-id='
 let initialContext = process.argv.find(arg => arg.startsWith(CONTEXT_ARG))
-if (initialContext) {
+if (process.webContentsId) {
+  // set by sandbox renderer init script
+  initialContext = process.webContentsId
+} else if (initialContext) {
   initialContext = parseInt(initialContext.substr(CONTEXT_ARG.length), 10)
 } else {
-  // In sandbox we need to pull this from remote
+  // if not available, pull from remote
   initialContext = exports.getCurrentWebContents().getId()
 }