Fix the test for CHANNEL_MESSAGING in idl files
https://bugs.webkit.org/show_bug.cgi?id=108006

Patch by Laszlo Gombos <l.gombos@samsung.com> on 2013-01-26
Reviewed by Alexey Proskuryakov.

No new tests, this is a build fix.

* workers/WorkerContext.idl: Make sure ENABLE_CHANNEL_MESSAGING is
defined before using it to be consitent with other guards inside idl
files.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140918 268f45cc-cd09-0410-ab3c-d52691b4dbfc