Use flag name without 'no' prefix only if it exists

Before this change it was impossible to use flag names that start with "no", e.g. --noise_file

BUG=webrtc:8202

Review-Url: https://codereview.webrtc.org/3004363002
Cr-Original-Commit-Position: refs/heads/master@{#19678}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 31377a2a103f040c3c4f0b1947d69657233c21a2
1 file changed