Uses AddHeaderFromString instead of AddHeadersFromString, for code health.

So it replaced AddHeadersFromString with AddHeaderFromString
in the net_unittests.

Bug: 862175
Change-Id: I3296f86f44ef5fca4b54fc267fdf4550707a3148
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573420
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Yeol Park <peary2@gmail.com>
Cr-Commit-Position: refs/heads/master@{#652418}
1 file changed