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This is needed to keep CORS handling in DocumentThredableLoader consistent
with that in ResourceLoader.

Bug: 702845
Change-Id: Iee73902e0941fb234aca3c60d996f9c75634222d
Reviewed-on: https://chromium-review.googlesource.com/571343Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487411}