IndexedDB: Re-use Unicode character constants from WTF::Unicode

Don't duplicate character constants for ZWJ and ZWNJ in the keypath
parser - use the handy constants from WTF::Unicode.

R=dgrogan@chromium.org

Review URL: https://codereview.chromium.org/25921002

git-svn-id: svn://svn.chromium.org/blink/trunk@158853 bbb929c8-8fbe-4397-9dbb-9b2b20218538