Use IDL argument default values in modules/indexeddb/

Replace bogus 'null' default values for DOMString arguments with the
actual default values from the spec, and remove the handling of null
strings in the C++ implementation.

Also replace [Default=Undefined] on some boolean arguments with the
default value 'false' from the spec, which has the same effect.

BUG=258153

Review URL: https://codereview.chromium.org/332613004

git-svn-id: svn://svn.chromium.org/blink/trunk@176930 bbb929c8-8fbe-4397-9dbb-9b2b20218538