[Cleanup] Remove muted state in MediaStreamTrack.

The state "muted" was removed as exposed interface in JavaScript in 2017.
Since the related bug has already been closed and marked as fixed, this
CL removes the state and the associated comment from the IDL since stale.

BUG=651414

Change-Id: I972c00cc4e59410e18391fad10c74af9d5131d3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1587881Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Armando Miraglia <armax@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654825}

third_party/blink/renderer/modules/mediastream/media_stream_track.idl

@@ -25,11 +25,8 @@
 
 // https://w3c.github.io/mediacapture-main/#mediastreamtrack
 
-// TODO(foolip): Remove or standardize MediaStreamTrackState "muted".
-// https://crbug.com/651414
 enum MediaStreamTrackState {
     "live",
-    "muted",
     "ended"
 };