Update .gitignore to properly ignore base/

TBR=dsinclair@chromium.org

Review-Url: https://codereview.chromium.org/2298603003
diff --git a/.gitignore b/.gitignore
index 8bb9d05..154b6f2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,4 @@
-/base/trace_event
+/base
 /build
 /buildtools
 /out