Expand is_linux to is_linux || is_chromeos.

Currently is_linux is set to true on Chrome OS build,
but it is planned to be set false. This CL is the preparation
to keep the compatibility.

Bug: chromium:1110266
Test: Build locally.
Change-Id: Ic79a202b0b3baeff157955cd03a07556bfb958a8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/183860
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#32073}

modules/audio_coding/BUILD.gn

@@ -483,7 +483,7 @@ rtc_library("isac_c") {
     "codecs/isac/main/source/transform.c",
   ]
 
-  if (is_linux) {
+  if (is_linux || is_chromeos) {
     libs = [ "m" ]
   }