Add top-level webkit/ directory to .gitignore.

Layout tests generate results in the webkit/ folder which was recently
removed from the chromium tree.

BUG=488288

Review URL: https://codereview.chromium.org/1137683004

Cr-Commit-Position: refs/heads/master@{#330431}

.gitignore

@@ -429,6 +429,8 @@ v8.log
 /ui/surface/surface.xml
 /ui/surface/surface_gpu_tests.xml
 /v8
+# Results directory for run-webkit-tests. http://crbug.com/488288
+/webkit
 /webpagereplay_logs/
 /win8/delegate_execute/delegate_execute_version_resources.xml
 /win8/metro_driver/metro_driver_version_resources.xml