gn: Support build directories outside the source tree.

tools/gn/source_dir.h

Index: tools/gn/source_dir.h
diff --git a/tools/gn/source_dir.h b/tools/gn/source_dir.h
index 719579b15ede8f07eb0fdbd248dbb3ef6e6012ed..ad9acfb9e3dbb20a8a60ccaeabe5c6fefcc940db 100644
--- a/tools/gn/source_dir.h
+++ b/tools/gn/source_dir.h
@@ -38,15 +38,13 @@ class SourceDir {
   // 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.