gn: Support build directories outside the source tree.

tools/gn/ninja_target_writer.cc

Index: tools/gn/ninja_target_writer.cc
diff --git a/tools/gn/ninja_target_writer.cc b/tools/gn/ninja_target_writer.cc
index 870cbb6d2eab11f8c88e3bf9ee1615796ef982ba..3ecba00141c5a37e5250d927e0db9ee5b3dd6e35 100644
--- a/tools/gn/ninja_target_writer.cc
+++ b/tools/gn/ninja_target_writer.cc
@@ -28,7 +28,9 @@ NinjaTargetWriter::NinjaTargetWriter(const Target* target,
     : settings_(target->settings()),
       target_(target),
       out_(out),
-      path_output_(settings_->build_settings()->build_dir(), ESCAPE_NINJA) {
+      path_output_(settings_->build_settings()->build_dir(),
+                   settings_->build_settings()->root_path_utf8(),
+                   ESCAPE_NINJA) {
 }
 
 NinjaTargetWriter::~NinjaTargetWriter() {
@@ -175,7 +177,8 @@ OutputFile NinjaTargetWriter::WriteInputDepsStampAndGetDep(
 
   OutputFile input_stamp_file(
       RebaseSourceAbsolutePath(GetTargetOutputDir(target_).value(),
-                               settings_->build_settings()->build_dir()));
+                               settings_->build_settings()->build_dir(),
+                               settings_->build_settings()->root_path_utf8()));
   input_stamp_file.value().append(target_->label().name());
   input_stamp_file.value().append(".inputdeps.stamp");