Fix gitignore to match DEPS for third_party/robolectric.

R=etiennej@chromium.org

Review URL: https://codereview.chromium.org/1416263002 .
diff --git a/.gitignore b/.gitignore
index 88723c7..061014e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -60,7 +60,7 @@
 /third_party/pyelftools/
 /third_party/pywebsocket/src/
 /third_party/requests/src/
-/third_party/robolectric/src/
+/third_party/robolectric/lib/
 /third_party/skia/
 /third_party/smhasher/src/
 /third_party/snappy/src/