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}

rtc_base/BUILD.gn

@@ -966,7 +966,7 @@ rtc_library("rtc_base") {
     deps += [ "system:cocoa_threading" ]
   }
 
-  if (is_linux) {
+  if (is_linux || is_chromeos) {
     libs += [
       "dl",
       "rt",