(core) Fix problem with localStorage in some cross-origin embed situations

Summary:
- Handle the possibility that any access to localStorage causes error.
- Move getStorage() and getSessionStorage() safe functions to a separate file.
- Use these safe functions in more places.

Test Plan:
Added a test case, using a webdriver instance that blocks third-party cookies,
to enforce third-party restrictions. Added to gristUtil a way to override the
webdriver instance.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3719

app/client/lib/sessionObs.ts

@@ -4,6 +4,7 @@
  */
 import {safeJsonParse} from 'app/common/gutil';
 import {IDisposableOwner, Observable} from 'grainjs';
+import {getSessionStorage} from 'app/client/lib/storage';
 
 export interface SessionObs<T> extends Observable<T> {
   pauseSaving(yesNo: boolean): void;
@@ -45,14 +46,15 @@ export function createSessionObs<T>(
     return value === _default || !isValid(value) ? null : JSON.stringify(value);
   }
   let _pauseSaving = false;
-  const obs = Observable.create<T>(owner, fromString(window.sessionStorage.getItem(key)));
+  const storage = getSessionStorage();
+  const obs = Observable.create<T>(owner, fromString(storage.getItem(key)));
   obs.addListener((value: T) => {
     if (_pauseSaving) { return; }
     const stored = toString(value);
     if (stored == null) {
-      window.sessionStorage.removeItem(key);
+      storage.removeItem(key);
     } else {
-      window.sessionStorage.setItem(key, stored);
+      storage.setItem(key, stored);
     }
   });
   return Object.assign(obs, {pauseSaving(yesNo: boolean) { _pauseSaving = yesNo; }});