Make some ScriptValue references const in modules/indexeddb

These functions are called from the bindings layer, so the non-const
references bind to its local variables, and thus limit what we can do
in the generated code; the parameter value must be an lvalue.

There was no obvious reason why these references should be non-const.

Review URL: https://codereview.chromium.org/497033002

git-svn-id: svn://svn.chromium.org/blink/trunk@180883 bbb929c8-8fbe-4397-9dbb-9b2b20218538