[Autofill] Remove warning for CC autofill on secure pages with mixed passive content

Test page: https://420fb7a0cffed907f3562bad84d357ad43f54dbd.googledrive.com/host/0B3xzZ-vFr2LRWVFBaWl2V0M4NFk

BUG=574839
TEST=manual

Review URL: https://codereview.chromium.org/1659463003

Cr-Commit-Position: refs/heads/master@{#372784}
3 files changed