Add lots of testonly annotations to the GN build.

BUG=
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Review URL: https://codereview.chromium.org/547793002

Cr-Commit-Position: refs/heads/master@{#293774}
diff --git a/ui/message_center/BUILD.gn b/ui/message_center/BUILD.gn
index 96315b5..536bcfc0 100644
--- a/ui/message_center/BUILD.gn
+++ b/ui/message_center/BUILD.gn
@@ -147,6 +147,7 @@
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "fake_message_center.h",
     "fake_message_center.cc",