GN: add default target.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 5660b4216044dded181c3ec70e55d031c06bef56..f5bfe7665869cef7b3beb94e22c22bddb279c716 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,7 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-group("root") {
+# This target will be built if no target is specified when invoking ninja.
+group("default") {
   testonly = true
 
   deps = [
@@ -17,3 +18,11 @@ group("root") {
     deps += [ "//tools/xdisplaycheck" ]
   }
 }
+
+# Deprecated name for the default build target.
+group("root") {
+  testonly = true
+  deps = [
+    ":default",
+  ]
+}