Motivation: Since I am activly refactoring SkXPS, I will be needing to
add and subtract sources. This is the only way to keep chrome and skia
in sync.
TODO(halcanry): follow up in Chrome build files.
BUG=skia:3495
Change-Id: I2bfcbfce7628c34f9a623c6fa806374a10cdbb75
Reviewed-on: https://skia-review.googlesource.com/6350
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
