Fix saving at webworker (#2647)

BUG

This PR is to fix the bug while saving models at webworker env. (#2643)

Note: Under webworker, we can only save models via IndexedDB. Maybe we should mention it at the docs?

tfjs-core/src/io/indexed_db.ts

@@ -57,7 +57,7 @@ function getIndexedDBFactory(): IDBFactory {
         'is not a web browser.');
   }
   // tslint:disable-next-line:no-any
-  const theWindow: any = window;
+  const theWindow: any = window || self;
   const factory = theWindow.indexedDB || theWindow.mozIndexedDB ||
       theWindow.webkitIndexedDB || theWindow.msIndexedDB ||
       theWindow.shimIndexedDB;

tfjs-core/src/io/local_storage.ts

@@ -38,6 +38,7 @@ const MODEL_METADATA_SUFFIX = 'model_metadata';
  */
 export function purgeLocalStorageArtifacts(): string[] {
   if (!env().getBool('IS_BROWSER') ||
+      typeof window === 'undefined' ||
       typeof window.localStorage === 'undefined') {
     throw new Error(
         'purgeLocalStorageModels() cannot proceed because local storage is ' +
@@ -119,7 +120,8 @@ export class BrowserLocalStorage implements IOHandler {
 
   constructor(modelPath: string) {
     if (!env().getBool('IS_BROWSER') ||
-        typeof window.localStorage === 'undefined') {
+          typeof window === 'undefined' ||
+          typeof window.localStorage === 'undefined') {
       // TODO(cais): Add more info about what IOHandler subtypes are
       // available.
       //   Maybe point to a doc page on the web and/or automatically determine
@@ -307,6 +309,7 @@ export class BrowserLocalStorageManager implements ModelStoreManager {
         env().getBool('IS_BROWSER'),
         () => 'Current environment is not a web browser');
     assert(
+        typeof window === 'undefined' ||
         typeof window.localStorage !== 'undefined',
         () => 'Current browser does not appear to support localStorage');
     this.LS = window.localStorage;