Replace request.onsuccess with trans.oncomplete for writes to IndexedDB

As per tweets with @PaulKinlan.  https://twitter.com/Paul_Kinlan/status/506818925834018817

I'm pretty sure I got all the languages but probably would double checking...  Incidentally, this didn't *seem* to make a difference until the latest version of Chrome.

[Also I have just submitted this too:- https://developers.google.com/open-source/cla/individual?csw=1]