Update the .gitignore file to skip LayoutTests/w3c.

TBR=jamesr@chromium.org
BUG=242329

Review URL: https://codereview.chromium.org/101393007

git-svn-id: svn://svn.chromium.org/blink/trunk@164261 bbb929c8-8fbe-4397-9dbb-9b2b20218538