Exempt Save-Data header from CORS preflight request check

Save-Data header is added by Chrome when Data saver feature is enabled.
This header should be treated as standard header and should be exempted
from CORS checks to trigger preflight request.

BUG=584889

Review URL: https://codereview.chromium.org/1698973002

Cr-Commit-Position: refs/heads/master@{#376410}
4 files changed