Catch possible exceptions when accessing window.localStorage

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/899

This is a code review of previous commits related to
the above issue.

platform/chromium/vapi-common.js

@@ -246,6 +246,7 @@ vAPI.localStorage = {
         this.cache = new Promise(resolve => {
             browser.storage.local.get('localStorage', bin => {
                 this.cache = undefined;
+                try {
                     if (
                         bin instanceof Object === false ||
                         bin.localStorage instanceof Object === false
@@ -256,13 +257,11 @@ vAPI.localStorage = {
                             const key = ls.key(i);
                             this.cache[key] = ls.getItem(key);
                         }
-                    //ls.clear();
                         browser.storage.local.set({ localStorage: this.cache });
                     } else {
-                    try {
                         this.cache = bin.localStorage;
-                    } catch(ex) {
                     }
+                } catch(ex) {
                 }
                 if ( this.cache instanceof Object === false ) {
                     this.cache = {};