Refactor IDBIndex::get/getKey

These methods have the same implementation except for one boolean flag, so add a helper method for this.

Review URL: https://codereview.chromium.org/128683004

git-svn-id: svn://svn.chromium.org/blink/trunk@164706 bbb929c8-8fbe-4397-9dbb-9b2b20218538