IDL compiler: simplify type conversion in Event Constructor

The current Dictionary::ConversionContext.withAttributes method is
code-generator unfriendly (no need for overloading, since we're generating
the code), and confusingly named.

This removes the unnecessary complexity of overloading, and renames
it to the clearer ConversionContext.conversionType.

Review URL: https://codereview.chromium.org/99333017

git-svn-id: svn://svn.chromium.org/blink/trunk@163704 bbb929c8-8fbe-4397-9dbb-9b2b20218538