[ObjC ARC] Converts ios/chrome/browser/ui/ntp:ntp_header to ARC.

Automatically generated ARCMigrate commit
Notable issues:None
BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2891563003
Cr-Commit-Position: refs/heads/master@{#475921}
5 files changed