Build WebRTC without support for the iLBC audio codec

It got enabled by this WebRTC commit
[https://webrtc-review.googlesource.com/c/src/+/9401], and will go
away again when bugs.webrtc.org/8396 is fixed, but we don't want to
wait, so we explicitly ask WebRTC to build without iLBC support.

I verified manually that with this change, the size of the `chrome`
binary in my opt Linux build goes from 189060824 to 188985648 bytes---a
drop of 73.4 kB, which is just the sort of effect I was expecting.

BUG=chromium:781027, webrtc:8396

Change-Id: I08301a1b4ea5994fb02ebaeffc6ae67ce3ec0d50
Reviewed-on: https://chromium-review.googlesource.com/753486
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Karl Wiberg <kwiberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514069}

• .gn[diff]