Fix SysStrings unit test failure on Chromecast

base/test/BUILD.gn

Index: base/test/BUILD.gn
diff --git a/base/test/BUILD.gn b/base/test/BUILD.gn
index 120159ee89576c9a52213cfb549ba922acbe95e9..b5984c6badaad164e5e043b5c05800f3c61d30ca 100644
--- a/base/test/BUILD.gn
+++ b/base/test/BUILD.gn
@@ -127,6 +127,13 @@ source_set("test_support") {
       "scoped_locale.h",
     ]
   }
+
+  # Allow more direct string conversions on platforms with native utf8
+  # strings
+  if (is_mac || is_ios || is_chromeos) {
+    defines += [ "SYSTEM_NATIVE_UTF8" ]
+  }
+
   if (is_ios) {
     # iOS uses its own unit test launcher.
     sources -= [ "launcher/unit_test_launcher.cc" ]