Simply V8 wrapper generation for Event objects

When trying to determine what kind of wrapper to produce, we'd iterate
over core event types (exiting early) then try iterating over module
event types. If the latter yielded an empty handle we'd wrap the
object as just a V8Event, which is bogus. This could happen in a
worker if the object was indeed a module event type, but the worker
had been asynchronously stopped and v8 was politely failing to produce
a wrapper.

Remove the fallback case, and instead assert if we make it as far as
module event type iteration and no match is found.

BUG=395411
R=abarth,tasak

Review URL: https://codereview.chromium.org/424813007

git-svn-id: svn://svn.chromium.org/blink/trunk@179340 bbb929c8-8fbe-4397-9dbb-9b2b20218538