Use default header extension map in rtc_event_log2text

Use defaults if the header extension map is missing from the config.

BUG=webrtc:6399

Review-Url: https://codereview.webrtc.org/2983283002
Cr-Original-Commit-Position: refs/heads/master@{#19267}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 007d56229adb8933960bfb08af1090ec20d48072
2 files changed