IDL compiler: type conversion for event constructor

This adds .setTypeConversion() to conversionContext.
This completes [EventConstructor]!

Generated code simplified in earlier:
IDL compiler: simplify type conversion in Event Constructor
https://codereview.chromium.org/99333017/

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/107463003

git-svn-id: svn://svn.chromium.org/blink/trunk@163748 bbb929c8-8fbe-4397-9dbb-9b2b20218538