Expand is_linux to is_linux || is_chromeos.

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

Bug: chromium:1110266
Test: Built locally.
Change-Id: I275a6a845eb9a5762e6dcb5be7d2f796916af51b
diff --git a/BUILD.gn b/BUILD.gn
index 61a5685..901ed1e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -128,7 +128,7 @@
       }
     } else if (is_mac || is_ios) {
       defines += [ "MACHO" ]
-    } else if (is_linux || is_android || is_fuchsia) {
+    } else if (is_linux || is_android || is_fuchsia || is_chromeos) {
       defines += [ "ELF" ]
     }
   }