Enforce WARN_UNUSED_RESULT attribute on Windows

components/storage_monitor/BUILD.gn

Index: components/storage_monitor/BUILD.gn
diff --git a/components/storage_monitor/BUILD.gn b/components/storage_monitor/BUILD.gn
index ccb33cea904163e1565fd1b5a57c507d1143e315..4a72f79041333e43390b5cf87cf4ffc88b31ca76 100644
--- a/components/storage_monitor/BUILD.gn
+++ b/components/storage_monitor/BUILD.gn
@@ -86,6 +86,7 @@ source_set("storage_monitor") {
 
 # GYP version: components/storage_monitor.gypi:storage_monitor_test_support
 source_set("test_support") {
+  testonly = true
   sources = [
     "mock_removable_storage_observer.cc",
     "mock_removable_storage_observer.h",
@@ -113,6 +114,12 @@ source_set("test_support") {
       "//device/media_transfer_protocol:mtp_storage_info_proto",
     ]
   }
+
+  if (is_win) {
+    deps = [
+      "//testing/gtest",
+    ]
+  }
 }
 
 source_set("unit_tests") {