Refactor GN source expansions.

tools/gn/source_file.h

Index: tools/gn/source_file.h
diff --git a/tools/gn/source_file.h b/tools/gn/source_file.h
index 83ce2b59d50bc182e205e1d0bac9fd5d9ed38c5e..0c1508e5255e4a0d1de434e6f88c7d5f82f50326 100644
--- a/tools/gn/source_file.h
+++ b/tools/gn/source_file.h
@@ -18,11 +18,17 @@ class SourceDir;
 // ends in one.
 class SourceFile {
  public:
+  enum SwapIn { SWAP_IN };
+
   SourceFile();
 
   // Takes a known absolute source file. Always begins in a slash.
   explicit SourceFile(const base::StringPiece& p);
 
+  // Constructs from the given string by swapping in the contents of the given
+  // value. The value will be the empty string after this call.
+  SourceFile(SwapIn, std::string* value);
+
   ~SourceFile();
 
   bool is_null() const { return value_.empty(); }