IndexedDB: Fixed threading bugs with use of AtomicStrings.
    
Some IndexedDB classes (IDBCursor, IDBRequest, IDBTransaction, and
IDBVersionChangeEvent) were using static local AtomicStrings on different
threads. This is safe, as long as these static strings are properly
created - which they were not. Switching to an initialization mechanism
which mirrors the Core static strings fixes this.

BUG=393728

Review URL: https://codereview.chromium.org/426063010

git-svn-id: svn://svn.chromium.org/blink/trunk@180205 bbb929c8-8fbe-4397-9dbb-9b2b20218538