Simplify Dictionary::ConversionContext

- Nobody uses the forConstructor().
- m_interfaceName and m_methodName are passed, but nobody uses them.

BUG=433174

Review URL: https://codereview.chromium.org/724183004

git-svn-id: svn://svn.chromium.org/blink/trunk@185360 bbb929c8-8fbe-4397-9dbb-9b2b20218538