gn: Support build directories outside the source tree.

tools/gn/source_dir.h

 // 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.
 
 #ifndef TOOLS_GN_SOURCE_DIR_H_
 #define TOOLS_GN_SOURCE_DIR_H_
 
 #include <algorithm>
 #include <string>
 
@@ skipping 20 matching lines @@
   // Swaps the given string in without copies. The given string will be empty
   // after this call.
   SourceDir(SwapIn, std::string* s);
   ~SourceDir();
 
   // Resolves a file or dir name relative to this source directory. Will return
   // an empty SourceDir/File on error. Empty input is always an error (it's
   // possible we should say ResolveRelativeDir vs. an empty string should be
   // the source dir, but we require "." instead).
   //
-  // If source_root is supplied, these functions will additionally handle the
+  // If |source_root| is non-empty, these functions will additionally handle the
   // case where the input is a system-absolute but still inside the source
   // tree. This is the case for some external tools.
-  SourceFile ResolveRelativeFile(
-      const base::StringPiece& p,
-      const base::StringPiece& source_root = base::StringPiece()) const;
-  SourceDir ResolveRelativeDir(
-      const base::StringPiece& p,
-      const base::StringPiece& source_root = base::StringPiece()) const;
+  SourceFile ResolveRelativeFile(const base::StringPiece& p,
+                                 const base::FilePath& source_root) const;
+  SourceDir ResolveRelativeDir(const base::StringPiece& p,
+                               const base::FilePath& source_root) const;
 
   // Resolves this source file relative to some given source root. Returns
   // an empty file path on error.
   base::FilePath Resolve(const base::FilePath& source_root) const;
 
   bool is_null() const { return value_.empty(); }
   const std::string& value() const { return value_; }
 
   // Returns true if this path starts with a "//" which indicates a path
   // from the source root.
@@ skipping 57 matching lines @@
 }
 #endif  // COMPILER...
 
 }  // namespace BASE_HASH_NAMESPACE
 
 inline void swap(SourceDir& lhs, SourceDir& rhs) {
   lhs.swap(rhs);
 }
 
 #endif  // TOOLS_GN_SOURCE_DIR_H_