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 TestInitFromValue(const std::string& values, bool expected_return_code,
                       int expected_extra_info_spec) {
  int actual_info_spec;
  bool actual_return_code = GenerateInfoSpec(values, &actual_info_spec);
  EXPECT_EQ(expected_return_code, actual_return_code);
  if (expected_return_code)
    EXPECT_EQ(expected_extra_info_spec, actual_info_spec);
}

void TestInitFromValue(const std::string& values, bool expected_return_code,
                       int expected_extra_info_spec) {
  int actual_info_spec;
  bool actual_return_code = GenerateInfoSpec(values, &actual_info_spec);
  EXPECT_EQ(expected_return_code, actual_return_code);
  if (expected_return_code)
    EXPECT_EQ(expected_extra_info_spec, actual_info_spec);
}
