Remove --preserve-root from roll_aosp.sh This flag is not supported on mac and generates errors when trying to roll. BUG=b:150892231 Change-Id: I22d10a2b9b37610d409df4d72e0aba2672f7b192 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2152666 Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>