Revert "Build: Disable the iLBC audio codec by default"

This reverts commit f14d2a500d5daadc3354c8ccd2bf1619bc0782e9.

Reason for revert: low_bandwidth_perf_tests regression.
https://bugs.chromium.org/p/webrtc/issues/detail?id=11748#c28
Should be fine to reland as soon as we understand why iLBC affects
low_bandwidth_perf_tests

BUG=webrtc:11748

Original change's description:
> Build: Disable the iLBC audio codec by default
>
> webrtc.org no longer builds with the iLBC audio codec by default by
> setting rtc_include_ilbc to false in webrtc.gni.
>
> You can read more about iLBC at
>   https://webrtc.github.io/webrtc-org/license/ilbc-freeware/
>
> iLBC is not a mandatory-to-implement codec in WebRTC (and we are
> pretty sure it was never enabled in Chrome). This affects only
> native applications which can change the build flag again.
>
> BUG=webrtc:11740
>
> Change-Id: I513658a950100a90346370c9210806aee0f34607
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178395
> Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31599}

TBR=mbonadei@webrtc.org,kwiberg@webrtc.org,hta@webrtc.org,philipp.hancke@googlemail.com,courtneyfe@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: webrtc:11740
Change-Id: Iaa0c39487464d31661ac8c22c9e21b6cb79b645a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178907
Commit-Queue: Olga Sharonova <olka@webrtc.org>
Reviewed-by: Olga Sharonova <olka@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31685}

webrtc.gni

@@ -65,7 +65,7 @@ declare_args() {
   rtc_builtin_ssl_root_certificates = true
 
   # Include the iLBC audio codec?
-  rtc_include_ilbc = false
+  rtc_include_ilbc = true
 
   # Disable this to avoid building the Opus audio codec.
   rtc_include_opus = true