[GN] Add support to rebase_path to resolve paths above the source root.

tools/gn/filesystem_utils.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_FILESYSTEM_UTILS_H_
 #define TOOLS_GN_FILESYSTEM_UTILS_H_
 
 #include <string>
 
 #include "base/files/file_path.h"
@@ skipping 93 matching lines @@
 //
 // The source_root should be a base::FilePath converted to UTF-8. On Windows,
 // it should begin with a "C:/" rather than being our SourceFile's style
 // ("/C:/"). The source root can end with a slash or not.
 //
 // Note that this does not attempt to normalize slashes in the output.
 bool MakeAbsolutePathRelativeIfPossible(const base::StringPiece& source_root,
                                         const base::StringPiece& path,
                                         std::string* dest);
 
-// Collapses "." and sequential "/"s and evaluates "..".
-void NormalizePath(std::string* path);
+// Collapses "." and sequential "/"s and evaluates "..". |path| may be
+// system-absolute, source-absolute, or relative. If |path| is source-absolute
+// and |source_root| is non-empty, |path| may be system absolute after this
+// function returns, if |path| references the filesystem outside of
+// |source_root| (ex. path = "//.."). In this case on Windows, |path| will have
+// a leading slash. Otherwise, |path| will retain its relativity. |source_root|
+// must not end with a slash.
+void NormalizePath(std::string* path,
+                   const base::StringPiece& source_root = base::StringPiece());
 
 // Converts slashes to backslashes for Windows. Keeps the string unchanged
 // for other systems.
 void ConvertPathToSystem(std::string* path);
 
 // Takes a path, |input|, and makes it relative to the given directory
 // |dest_dir|. Both inputs may be source-relative (e.g. begins with
 // with "//") or may be absolute.
 //
 // If supplied, the |source_root| parameter is the absolute path to
@@ skipping 62 matching lines @@
 
 SourceDir GetTargetOutputDir(const Target* target);
 OutputFile GetTargetOutputDirAsOutputFile(const Target* target);
 SourceDir GetTargetGenDir(const Target* target);
 OutputFile GetTargetGenDirAsOutputFile(const Target* target);
 
 SourceDir GetCurrentOutputDir(const Scope* scope);
 SourceDir GetCurrentGenDir(const Scope* scope);
 
 #endif  // TOOLS_GN_FILESYSTEM_UTILS_H_