webkitRTCPeerConnection warning from bundled closure-compiler externs

[vapier]

starting with the v20200426 release, an error is always emitted from the bundled externs archive, even when run on a completely empty input.

repro:

$ closure-compiler --version
Closure Compiler (http://github.com/google/closure-compiler)
Version: v20200426
Built on: 2020-05-01 00:00

$ echo | closure-compiler --checks-only --jscomp_error=*
The compiler is waiting for input via stdin.
externs.zip//nonstandard_rtc.js:44: ERROR - [JSC_STRICT_INEXISTENT_PROPERTY] Property webkitRTCPeerConnection never defined on Window
var webkitRTCPeerConnection = RTCPeerConnection;
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 error(s), 0 warning(s)

[brad4d]

I'm working on fixing this.