Make GN --as=output relative to the root build directory.

tools/gn/commands.cc

Index: tools/gn/commands.cc
diff --git a/tools/gn/commands.cc b/tools/gn/commands.cc
index 3778d5bb6445574e563e4f6788e3280b4b99bda1..ed7a009d6af2c6f7a866d7d82e753b701d07760e 100644
--- a/tools/gn/commands.cc
+++ b/tools/gn/commands.cc
@@ -323,8 +323,6 @@ void PrintTargetsAsOutputs(bool indent,
   const BuildSettings* build_settings =
       targets[0]->settings()->build_settings();
 
-  SourceDir current_dir = SourceDirForCurrentDirectory(
-      build_settings->root_path());
   for (const Target* target : targets) {
     // Use the link output file if there is one, otherwise fall back to the
     // dependency output file (for actions, for example).
@@ -334,7 +332,8 @@ void PrintTargetsAsOutputs(bool indent,
 
     SourceFile output_as_source =
         output_file.AsSourceFile(build_settings);
-    std::string result = RebasePath(output_as_source.value(), current_dir,
+    std::string result = RebasePath(output_as_source.value(),
+                                    build_settings->build_dir(),
                                     build_settings->root_path_utf8());
     if (indent)
       OutputString("  ");