OOR-CORS: head.headers should be checked before calling response_code()

PreflightController still have one more issue that didn't check
headers existence before calling response_code().

Bug: 928394
Change-Id: I5841ba0b8d26c5d30b8eae257f6e4a88c6d158e3
Reviewed-on: https://chromium-review.googlesource.com/c/1457810
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630284}
3 files changed