Fix GYP DEPTH for libjingle isolate files

In https://review.webrtc.org/13679004/ the libjingle isolate
files in patch set #2 were not tested, which caused a failure when
6427 was committed. This fixes the talk/build/isolate.gypi with a
similar change.

BUG=343106
TEST=Successful local compile on Linux
TBR=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/14689004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6428 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/talk/build/isolate.gypi b/talk/build/isolate.gypi
index d1ff291..24f7ea1 100644
--- a/talk/build/isolate.gypi
+++ b/talk/build/isolate.gypi
@@ -113,6 +113,7 @@
 
         # Path variables are used to replace file paths when loading a .isolate
         # file
+        '--path-variable', 'DEPTH', '<(DEPTH)',
         '--path-variable', 'PRODUCT_DIR', '<(PRODUCT_DIR) ',
 
         '--config-variable', 'OS=<(OS)',