Updated the WebChannel JS API spec wrt metadata support. RELNOTES: n/a
Updated the WebChannel JS API spec wrt metadata support.
RELNOTES: n/a

PiperOrigin-RevId: 417163723
Change-Id: Iea6bc94d7309d6ad45307bdc483da0edb6c6deea

google/closure-library,google/closure-library,google/closure-library,google/closure-library,google/closure-library