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We want to get rid of the single argument AddHeader() method as the two
argument AddHeader() and SetHeader() methods are safer and more
flexible.

In this case, switching to SetHeader() to make clear that we only expect
one instance of each of the headers being set.

Bug: 1068194
Change-Id: Ieeeb96ea9b55e8ddb606ee64949d7b3271226aca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149686Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759926}