headless: Add a headless shell

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index bfc0fc88a448d04ee57ef55f41e753146e438ea4..ceaeadceb2a8639fd6b296874233c8ebfcaf97d2 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -646,6 +646,10 @@ group("gn_only") {
     deps -= [ "//mandoline:all" ]  # TODO(GYP)
   }
 
+  if (is_headless) {

[Peter Beverloo, 2015/10/27 14:35:51]

Does this need to be defined somewhere?

Documenting it in //tools/gn/docs/cookbook.md also seems appropriate.

+    deps += [ "//headless" ]
+  }
+
   if (use_libfuzzer) {
     deps += [ "//testing/libfuzzer:libfuzzer_main" ]
   }