gn format: support absolute file system paths

tools/gn/source_file.cc

Index: tools/gn/source_file.cc
diff --git a/tools/gn/source_file.cc b/tools/gn/source_file.cc
index 7744850d5c5966ea048f529641b5683ba5ebbe88..b3eb560bac5b93d8a8f411c60aab7d99174ba8a7 100644
--- a/tools/gn/source_file.cc
+++ b/tools/gn/source_file.cc
@@ -64,6 +64,8 @@ base::FilePath SourceFile::Resolve(const base::FilePath& source_root) const {
   }
 
   converted.assign(&value_[2], value_.size() - 2);
+  if (source_root.empty())
+    return UTF8ToFilePath(converted).NormalizePathSeparatorsTo('/');
   return source_root.Append(UTF8ToFilePath(converted))
       .NormalizePathSeparatorsTo('/');
 }