Added IndexedDB utility functions for reading and writing bookmarks

Summary:
These are utility function for reading from the IndexedDB and writing to it. This will be used for persistant storage of bookmarks.

The reason I chose IndexedDB over local storage is due to the poor efficiency of stringifying and parsing a long list of bookmarks everytime we read and write to local storage.

With IndexedDB we can modify a single value at a time.

Bookmarks are passed around as a Map, with the key being the uri's and the common names being the values. This allows me to check if a specified uri is in the Map in O(1) time, so that I can highlight the star icon in the UI with gold.

Reviewed By: jknoxville

Differential Revision: D16498744

fbshipit-source-id: 7c7af28bf4eb3fcc985a71dfd61ffbdb8481b6a6

flow-typed/indexedDB.js

@@ -0,0 +1,31 @@
+declare class DOMStringList {
+  +[key: number]: string;
+  +length: number;
+  item: number => string | null;
+  contains: string => boolean;
+}
+
+declare interface IDBDatabase extends EventTarget {
+  close(): void;
+  createObjectStore(
+    name: string,
+    options?: {
+      keyPath?: ?(string | string[]),
+      autoIncrement?: boolean,
+      ...
+    },
+  ): IDBObjectStore;
+  deleteObjectStore(name: string): void;
+  transaction(
+    storeNames: string | string[],
+    mode?: 'readonly' | 'readwrite' | 'versionchange',
+  ): IDBTransaction;
+  name: string;
+  version: number;
+  objectStoreNames: string[];
+  objectStoreNames: DOMStringList;
+  onabort: (e: any) => mixed;
+  onclose: (e: any) => mixed;
+  onerror: (e: any) => mixed;
+  onversionchange: (e: any) => mixed;
+}