Correct handling of return types for callbacks.

Correct handling of return types for callbacks.

Actually, all WebKit cares about is if callback threw an exception or not.
So return type of callbacks should be actually ignored.
As of now it is always bool for WebKit IDLs, but sometimes it gets overridden
to void by supplemental IDLs.

R=podivilov@chromium.org
BUG=3016
Committed: https://code.google.com/p/dart/source/detail?r=12440

[podivilov, 2012-09-17 15:59:03]

lgtm