refactor($pwa): using define API.

Author: ulivz

packages/@vuepress/plugin-pwa/index.js

@@ -8,17 +8,14 @@ module.exports = (options, context) => ({
     }, options)
   },
 
-  chainWebpack (config) {
+  define () {
     const { serviceWorker, updatePopup } = options
     const base = context.base || '/'
-
-    config.plugin('injections').tap(([options]) => [
-      Object.assign(options, {
-        SW_BASE_URL: JSON.stringify(base),
-        SW_ENABLED: !!serviceWorker,
-        SW_UPDATE_POPUP: updatePopup ? JSON.stringify(updatePopup) : false
-      })
-    ])
+    return {
+      SW_BASE_URL: base,
+      SW_ENABLED: !!serviceWorker,
+      SW_UPDATE_POPUP: updatePopup || false
+    }
   },
 
   // TODO support components option