bindings: Pass IDL dictionary references instead of pointers

It will be less error-prone to pass references instead of pointers.
Customers don't need to add ASSERT().

This is a follow-up CL of https://codereview.chromium.org/547383004/,
and no changes in behavior.

BUG=321462

Review URL: https://codereview.chromium.org/558283004

git-svn-id: svn://svn.chromium.org/blink/trunk@181871 bbb929c8-8fbe-4397-9dbb-9b2b20218538