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..d7415c9887bf574fc0a196fb9448844acb650556 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 +123,16 @@ 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.
-std::string RebaseSourceAbsolutePath(const std::string& input,
-                                     const SourceDir& dest_dir);
+// 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 must be the absolute path

[brettw, 2014/11/05 19:59:49]

"must be" -> "is the". Can you explain here what happens if this is supplied?
The current comment gives no reason why you would bother specifying this.

[zeuthen, 2014/11/07 19:24:12]

Done.

+// to the source root and not end in a slash.
+std::string RebaseSourceAbsolutePath(
+    const std::string& input,
+    const SourceDir& dest_dir,
+    const base::StringPiece& source_root = base::StringPiece());
 
 // Returns the given directory with no terminating slash at the end, such that
 // appending a slash and more stuff will produce a valid path.