Add isolate targets for libjingle

Add .isolate file for libjingle tests and and the necessary isolate.gypi file, similar to the change in
http://review.webrtc.org/2338004/

TEST=trybots passing.
I also ran build/gyp_chromium in a Chromium checkout
with third_party/libjingle/source/talk having this patch
applied to ensure GYP processing was still working.

BUG=1916
R=henrike@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2353005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4926 4adac7df-926f-26a2-2b94-8c16560cd09d
7 files changed