Remove a |readonly| member of MediaStreamTrack

Spec reference: http://w3c.github.io/mediacapture-main/getusermedia.html#attributes-1 - removal was in the April 6 editors' draft.

The attribute was never exposed in our IDL files.

BUG=598704

Review-Url: https://codereview.chromium.org/1915793002
Cr-Commit-Position: refs/heads/master@{#392373}
26 files changed