IndexedDB: Use leveldb::DestroyDB to delete backing stores

Rather than just blowing away the directory, use leveldb's dedicated
DestroyDB() to remove backing stores, which attempts to grab a lock.

Review URL: https://codereview.chromium.org/27048003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228768 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228768