Reference written files relatively when possible.

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 4f258e1d47c5bfe3a97b885d0e46326f1d5a43ff..47caad7520f3e3adc9180bbd9c4f6a51b6eab8d1 100644
--- a/tools/gn/filesystem_utils.h
+++ b/tools/gn/filesystem_utils.h
@@ -100,6 +100,13 @@ bool EnsureStringIsInOutputDir(const SourceDir& output_dir,
                                const ParseNode* origin,
                                Err* err);
 
+// If the file isn't in the dir, returns the empty string and sets the error.
+// Otherwise returns the short output name and leaves the error untouched.
+base::FilePath::StringType ComputeStringInOutputDir(const SourceDir& output_dir,
+                                                    const SourceFile& file,
+                                                    Err* err);
+
+
 // ----------------------------------------------------------------------------
 
 // Returns true if the input string is absolute. Double-slashes at the