Consolidate .gitignore entries for XCode projects.

Follow-up to feedback in https://codereview.chromium.org/24488003/

BUG=297962

Review URL: https://codereview.chromium.org/25039012

git-svn-id: svn://svn.chromium.org/blink/trunk@158567 bbb929c8-8fbe-4397-9dbb-9b2b20218538