Change minimum DTMF event duration to be 40 milliseconds

The current value of 100 milliseconds is the recommended default value
in https://w3c.github.io/webrtc-pc/#rtcdtmfsender; the actual minimum specified is 40 milliseconds.

BUG=webrtc:7163

Review-Url: https://codereview.webrtc.org/2699503002
Cr-Original-Commit-Position: refs/heads/master@{#17430}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 588101cb91468673c66cd84029c561e8ef7a9279
3 files changed