Use <(DEPTH) in derived_sources.gyp for saner paths

Per abarth's suggestion, makes GYP more legible and robust:
https://chromiumcodereview.appspot.com/15801003/#msg34

Review URL: https://chromiumcodereview.appspot.com/19476005

git-svn-id: svn://svn.chromium.org/blink/trunk@154751 bbb929c8-8fbe-4397-9dbb-9b2b20218538