Bug 1379982 - fix mach webrtc-gtest command on mac; r=chmanchester

We need to add webrtc-gtest to MacOS-files.in in order for it to be built
in the place that mach expects to find it.

MozReview-Commit-ID: 74kWZeE65R1

--HG--
extra : rebase_source : 06b923f17d65dd491e071a1c1f37ae1c736f66db

browser/app/macbuild/Contents/MacOS-files.in

@@ -9,5 +9,6 @@
 /pingsender
 /pk12util
 /ssltunnel
+/webrtc-gtest
 /xpcshell
 /XUL