gn: Support build directories outside the source tree.

tools/gn/commands.cc

Index: tools/gn/commands.cc
diff --git a/tools/gn/commands.cc b/tools/gn/commands.cc
index 0ad1bd77b1051dd1e803aa072cbb15564bd3ee9d..198e63f3398492c69fdb5fb5a7756d9f53556a22 100644
--- a/tools/gn/commands.cc
+++ b/tools/gn/commands.cc
@@ -58,7 +58,9 @@ const Target* ResolveTargetFromCommandLineString(
   Err err;
   Label label = Label::Resolve(SourceDirForCurrentDirectory(
                                    setup->build_settings().root_path()),
-                               default_toolchain, arg_value, &err);
+                               default_toolchain, arg_value,
+      setup->build_settings().root_path(),
+      &err);
   if (err.has_error()) {
     err.PrintToStdout();
     return NULL;
@@ -92,6 +94,7 @@ bool ResolveTargetsFromCommandLinePattern(
 
   Err err;
   LabelPattern pattern = LabelPattern::GetPattern(
+      setup->build_settings().root_path(),
       SourceDirForCurrentDirectory(setup->build_settings().root_path()),
       pattern_value,
       &err);