Add Mojo application test support to Chromium.

mojo/application/BUILD.gn

Index: mojo/application/BUILD.gn
diff --git a/mojo/application/BUILD.gn b/mojo/application/BUILD.gn
index 746f778db942342cb989fe943e93e5dccc423bbd..601391f02b6488b6a3c8f7414fb897dedcbc9f83 100644
--- a/mojo/application/BUILD.gn
+++ b/mojo/application/BUILD.gn
@@ -18,3 +18,11 @@ source_set("application") {
     "//mojo/environment:chromium",
   ]
 }
+
+source_set("test_support") {
+  testonly = true
+  sources = [ "application_test_main_chromium.cc" ]
+
+  public_deps = [ "//mojo/public/cpp/application:test_support" ]
+  deps = [ "//base" ]
+}