Expand is_linux to is_linux || is_chromeos.

Currently is_linux is set to true on Chrome OS build,
but it is planned to set it to false. This CL is the preparation
to keep the behavior even after the switching.

Bug: 1110266
Test: Built locally.
Change-Id: Ic4fca9914a62080c525baf39ec9e203d8bf95bfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2413652
Auto-Submit: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808148}

components/metrics/BUILD.gn

@@ -161,7 +161,7 @@ static_library("metrics") {
     sources += [ "drive_metrics_provider_ios.mm" ]
   }
 
-  if (is_linux) {
+  if (is_linux || is_chromeos) {
     sources += [
       "drive_metrics_provider_linux.cc",
       "system_memory_stats_recorder_linux.cc",