IndexedDB: Ignore duplicate transaction IDs sent by renderer

BUG=297556
R=alecflett@chromium.org,aedla@chromium.org

Review URL: https://codereview.chromium.org/24702002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226009 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226009