[filesapp] Add (primitive) storage to files SWA chrome.storage fake

Add actual storage to the SWA chrome.storage fake, to allow removal of
of more "if (window.isSWA) {...}" code from CL:2500642. Implementation
code derived from

  ui/file_manager/file_manager/test/js/chrome_api_test_impl.js

Bug: 1113981
Tbr: Bo
Change-Id: I5dafeb94616ef9e051f8a3fb829f1204f039c69b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2529270
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826162}

chromeos/components/file_manager/resources/file_manager_fakes.js

@@ -15,12 +15,64 @@ window.chrome.storage = {
     addListener() {},
   },
   sync: {
-    get() {},
-    set() {},
+    /** @private {!Object} store */
+    store_: {},
+    /**
+     * @param {!Array<string>|string} keys
+     * @param {function((!Object)} callback
+     */
+    get : (keys, callback) => {
+      const inKeys = Array.isArray(keys) ? keys : [keys];
+      const result = {};
+      inKeys.forEach(key => {
+        if (key in chrome.storage.sync.store_) {
+          result[key] = chrome.storage.sync.store_[key];
+        }
+      });
+      setTimeout(callback, 0, result);
+    },
+    /**
+     * @param {!Object<string>} items
+     * @param {function()=} opt_callback
+     */
+    set: (items, opt_callback) => {
+      for (const key in items) {
+        chrome.storage.sync.store_[key] = items[key];
+      }
+      if (opt_callback) {
+        setTimeout(opt_callback);
+      }
+    },
   },
   local: {
-    get() {},
-    set() {},
+    /** @private {!Object} store */
+    store_: {},
+    /**
+     * @param {!Array<string>|string} keys
+     * @param {function((!Object)} callback
+     */
+    get : (keys, callback) => {
+      const inKeys = Array.isArray(keys) ? keys : [keys];
+      const result = {};
+      inKeys.forEach(key => {
+        if (key in chrome.storage.local.store_) {
+          result[key] = chrome.storage.local.store_[key];
+        }
+      });
+      setTimeout(callback, 0, result);
+    },
+    /**
+     * @param {!Object<string>} items
+     * @param {function()=} opt_callback
+     */
+    set: (items, opt_callback) => {
+      for (const key in items) {
+        chrome.storage.local.store_[key] = items[key];
+      }
+      if (opt_callback) {
+        setTimeout(opt_callback);
+      }
+    },
   },
 };