IndexedDB: Remove temporary #define following Chromium update

To facilitate a behavior-changing two-sided patch, a public-facing
flag #define was introduced. Now that both sides have landed and
Chromium no longer needs flag, remove it.

BUG=100123
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/381453002

git-svn-id: svn://svn.chromium.org/blink/trunk@177760 bbb929c8-8fbe-4397-9dbb-9b2b20218538