Replace fixed string uses of AddHeaderFromString

Uses of AddHeaderFromString() with a static string may as well be
replaced with SetHeader(). Do so.

BUG=None

Review-Url: https://codereview.chromium.org/2236933005
Cr-Commit-Position: refs/heads/master@{#418161}
void MockNetworkTransaction::SetBeforeHeadersSentCallback(
    const BeforeHeadersSentCallback& callback) {}

void MockNetworkTransaction::SetBeforeHeadersSentCallback(
    const BeforeHeadersSentCallback& callback) {}
