gn: Support build directories outside the source tree.

tools/gn/filesystem_utils.h

Index: tools/gn/filesystem_utils.h
diff --git a/tools/gn/filesystem_utils.h b/tools/gn/filesystem_utils.h
index 20773cc7dd6a9e2354c943fb06211fca88a676ef..3622910ffb6d09ffaefe921507c2db739b334dfd 100644
--- a/tools/gn/filesystem_utils.h
+++ b/tools/gn/filesystem_utils.h
@@ -116,11 +116,6 @@ bool MakeAbsolutePathRelativeIfPossible(const base::StringPiece& source_root,
                                         const base::StringPiece& path,
                                         std::string* dest);
 
-// Converts a directory to its inverse (e.g. "/foo/bar/" -> "../../").
-// This will be the empty string for the root directories ("/" and "//"), and
-// in all other cases, this is guaranteed to end in a slash.
-std::string InvertDir(const SourceDir& dir);
-
 // Collapses "." and sequential "/"s and evaluates "..".
 void NormalizePath(std::string* path);
 
@@ -128,10 +123,13 @@ void NormalizePath(std::string* path);
 // for other systems.
 void ConvertPathToSystem(std::string* path);
 
-// Takes a source-absolute path (must begin with "//") and makes it relative
-// to the given directory, which also must be source-absolute.
+// 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. The |source_root| parameter must be
+// the absolute path to the source root.
 std::string RebaseSourceAbsolutePath(const std::string& input,
-                                     const SourceDir& dest_dir);
+                                     const SourceDir& dest_dir,
+                                     const base::FilePath& source_root);
 
 // Returns the given directory with no terminating slash at the end, such that
 // appending a slash and more stuff will produce a valid path.