gn: Support build directories outside the source tree.

tools/gn/path_output.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/path_output.h"
 
+#include "base/strings/string_util.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/output_file.h"
 #include "tools/gn/string_utils.h"
 
-PathOutput::PathOutput(const SourceDir& current_dir, EscapingMode escaping)
+PathOutput::PathOutput(const SourceDir& current_dir,
+                       const base::FilePath& source_root, EscapingMode escaping)
     : current_dir_(current_dir) {
-  CHECK(current_dir.is_source_absolute())
-      << "Currently this only supports writing to output directories inside "
-         "the source root. There needs to be some tweaks to PathOutput to make "
-         "doing this work correctly.";
-  inverse_current_dir_ = InvertDir(current_dir_);
-
+  inverse_current_dir_ =
+      RebaseSourceAbsolutePath("//", current_dir, source_root);
+  if (!EndsWithSlash(inverse_current_dir_))
+    inverse_current_dir_.push_back('/');
   options_.mode = escaping;
 }
 
 PathOutput::~PathOutput() {
 }
 
 void PathOutput::WriteFile(std::ostream& out, const SourceFile& file) const {
   WritePathStr(out, file.value());
 }
 
@@ skipping 125 matching lines @@
     // it's system-absolute.
 #if defined(OS_WIN)
     // On Windows, trim the leading slash, since the input for absolute
     // paths will look like "/C:/foo/bar.txt".
     EscapeStringToStream(out, str.substr(1), options_);
 #else
     EscapeStringToStream(out, str, options_);
 #endif
   }
 }