gn: Do not include "webrtc" in Chromium/iOS builds.
gn: Do not include "webrtc" in Chromium/iOS builds.

It's not part of the gyp build either, and `ninja -C out/gnios webrtc`
causes several build errors, at least in Chromium builds (which means
gn/ios bots building 'all' are broken).

BUG=chromium:633316
R=kjellander@webrtc.org

Review URL: https://codereview.webrtc.org/2205533003 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13609}

TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc