Improve handling of dictionary conversions.

When generating bindings for the dictionaries that Event constructors
take, emit code that enforce the required conversion step as per
WebIDL. i.e., if the property fails to convert, emit the required
(TypeError) exception and fail.

Previously these were mapped to empty/zero/null instead.

R=
BUG=323036

Review URL: https://codereview.chromium.org/85263002

git-svn-id: svn://svn.chromium.org/blink/trunk@162859 bbb929c8-8fbe-4397-9dbb-9b2b20218538