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 TestTransactionConsumer::DidFinish(int result) {
  state_ = DONE;
  error_ = result;
  if (--quit_counter_ == 0)
    base::MessageLoop::current()->QuitWhenIdle();
}

void TestTransactionConsumer::DidFinish(int result) {
  state_ = DONE;
  error_ = result;
  if (--quit_counter_ == 0)
    base::MessageLoop::current()->QuitWhenIdle();
}
