GitLab

Since the constructor of RTCDTMFToneChangeEvent is exposed,
creating such events with other names than "tonechange" is possible.
No reason to discriminate against such.

Also adds tests for constructor.

Bug: chromium:822799
Change-Id: I4b36f3094acee200dd4200c4d24f6b46e10a06e6
Reviewed-on: https://chromium-review.googlesource.com/992038Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547742}