GN: add default target.

Previously, invoking ninja without specifying a target would cause it to try to
build a number of unused and broken targets carried over from Chrome.  This was
worked around by building an explicit "root" target that contained only the
targets needed by Mojo.  The default (no target) build is now equivalent to
"root".  The "root" target still exists, but is now deprecated.

BUG=https://code.google.com/p/chromium/issues/detail?id=401761
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/809583002
diff --git a/BUILD.gn b/BUILD.gn
index 5660b42..f5bfe76 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 @@
     deps += [ "//tools/xdisplaycheck" ]
   }
 }
+
+# Deprecated name for the default build target.
+group("root") {
+  testonly = true
+  deps = [
+    ":default",
+  ]
+}