Reference written files relatively when possible.

tools/gn/filesystem_utils.cc

Index: tools/gn/filesystem_utils.cc
diff --git a/tools/gn/filesystem_utils.cc b/tools/gn/filesystem_utils.cc
index 663e6ac3f0c188d708a366d3326eb273605d4130..1f8fe81d2cf2668c10a2de2155f50351a4c18c00 100644
--- a/tools/gn/filesystem_utils.cc
+++ b/tools/gn/filesystem_utils.cc
@@ -332,6 +332,32 @@ bool EnsureStringIsInOutputDir(const SourceDir& output_dir,
   return false;
 }
 
+base::FilePath::StringType ComputeStringInOutputDir(const SourceDir& output_dir,
+                                           const SourceFile& output_file,
+                                           Err* err) {
+  if (output_file.is_source_absolute() != output_dir.is_source_absolute()) {
+    *err = Err(nullptr,
+        "Cannot determine if output file is in output directory",
+        "Absolute files/dirs are incomparable with source rooted ones.");
+    return base::FilePath::StringType();
+  }
+
+  const std::string& dir_str = output_dir.value();
+  const std::string& str = output_file.value();
+  if (str.compare(0, dir_str.length(), dir_str) != 0 ||
+      str.length() <= dir_str.length() ||
+      IsSlash(str[dir_str.length()])) {
+    *err = Err(nullptr, "File is not inside output directory.",
+        "The given file should be in the output directory. Normally you would "
+        "specify\n\"$target_out_dir/foo\" or "
+        "\"$target_gen_dir/foo\". I interpreted this as\n\""
+        + str + "\".");
+    return base::FilePath::StringType();
+  }
+  return base::FilePath::StringType(str.c_str() + dir_str.length(),
+                                    str.length() - dir_str.length());
+}
+
 bool IsPathAbsolute(const base::StringPiece& path) {
   if (path.empty())
     return false;