Use $root:default as a "default" rule

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index 2c2ba38526a3bf831b0ce37ef7e20b84f8aed5ec..6f23a997dc0cf5c29ccd2b51d414d17b7b48478f 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -95,7 +95,7 @@ Variables
 
   root [optional]
       Label of the root build target. The GN build will start by loading the
-      build file containing this target name. This defaults to "//:" which will
+      build file containing this target name. This defaults to label which will

[brettw, 2017/04/19 19:49:25]

This new text isn't gramatical. I think the old text is probably fine. It's what
actually happens, and I don't think people are commonly reading this
documentation and setting the root such that they would get confused by it.

[Petr Hosek, 2017/04/19 20:20:26]

Done.

[Dirk Pranke, 2017/04/19 23:01:27]

FWIW, I still find it confusing.

       cause the file //BUILD.gn to be loaded.
 
   secondary_source [optional]
@@ -698,6 +698,7 @@ bool Setup::RunConfigFile() {
 bool Setup::FillOtherConfig(const base::CommandLine& cmdline) {
   Err err;
   SourceDir current_dir("//");
+  Label root_target_label(current_dir, "");
 
   // Secondary source path, read from the config file if present.
   // Read from the config file if present.
@@ -720,8 +721,7 @@ bool Setup::FillOtherConfig(const base::CommandLine& cmdline) {
       return false;
     }
 
-    Label root_target_label =
-        Label::Resolve(current_dir, Label(), *root_value, &err);
+    root_target_label = Label::Resolve(current_dir, Label(), *root_value, &err);
     if (err.has_error()) {
       err.PrintToStdout();
       return false;
@@ -729,6 +729,7 @@ bool Setup::FillOtherConfig(const base::CommandLine& cmdline) {
 
     root_build_file_ = Loader::BuildFileForLabel(root_target_label);
   }
+  build_settings_.SetRootTargetLabel(root_target_label);
 
   // Build config file.
   const Value* build_config_value =