[build] Add `is_chromeos = false` to BUILDCONFIG.gn

This accommodates the addition of this variable to Chromium ICU's BUILD.gn.
See https://fuchsia-review.googlesource.com/c/third_party/icu/+/279592.

I18N-14 #comment

Change-Id: I605b379b2a5caf3aaa303e4554008d69f893f84a
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 24f044b..3de3743 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -92,6 +92,7 @@
 }
 
 is_android = false
+is_chromeos = false
 is_fuchsia = false
 is_fuchsia_host = false
 is_ios = false