enable gold for chromeos build.

When I try to enable thinlto on ChromeOS build, I hit this assert.
This CL fixes that.

BUG=None
TEST=USE="thinlto" emerge-terra chromeos-chrome, the assertion is
    gone.

Review-Url: https://codereview.chromium.org/2960273002
Cr-Commit-Position: refs/heads/master@{#488345}

build/config/compiler/BUILD.gn

@@ -481,7 +481,7 @@ config("compiler") {
   # TODO(pcc): Make this conditional on is_official_build rather than on gn
   # flags for specific features.
   if (!is_debug && (allow_posix_link_time_opt || is_cfi) && !is_nacl) {
-    assert(use_lld || is_chromeos, "gold plugin only supported with ChromeOS")
+    assert(use_lld || target_os == "chromeos", "gold plugin only supported with ChromeOS")
 
     if (use_thin_lto) {
       cflags += [ "-flto=thin" ]