Fixed regression: Clickhide functionality broken

The clickhide feature was broken due to the recent Safari related changes. This has already been addressed in http://codereview.adblockplus.org/5762529972191232/ but not completely fixed yet.

To fix the issue, usage of sendMessage/onMessage had to be reverted to sendRequest/onRequest due to differing behaviors (as reported at https://code.google.com/p/chromium/issues/detail?id=321074).

[Thomas Greiner, 2013-11-19 12:23:26]

Apparently, this is not a bug but a feature. :D Adapted changes to make it work
with sendMessage/onMessage after all.

[Sebastian Noack, 2013-11-19 13:25:31]

I wonder whether it would be better to always return true from the event
listener, in order to have a consistent behavior between Chrome and Safari.

[Wladimir Palant, 2013-11-19 13:48:25]

On 2013/11/19 13:25:31, sebastian wrote:
> I wonder whether it would be better to always return true from the event
> listener, in order to have a consistent behavior between Chrome and Safari.

No, that would introduce memory leaks - we don't always call the callback.
Properly indicating that the message will be handled asynchronously is actually
necessary, even if it will be ignored in Safari.

LGTM