[WIP] make GURL::path() return a StringPiece

extensions/common/file_util.cc

Index: extensions/common/file_util.cc
diff --git a/extensions/common/file_util.cc b/extensions/common/file_util.cc
index 91d49d0319bebb549d36d938d5d3a51782b9eae1..d7b0deed567e2d0812ce7ca4e0cd54c59f360cf1 100644
--- a/extensions/common/file_util.cc
+++ b/extensions/common/file_util.cc
@@ -436,13 +436,13 @@ void DeleteFile(const base::FilePath& path, bool recursive) {
 }
 
 base::FilePath ExtensionURLToRelativeFilePath(const GURL& url) {
-  std::string url_path = url.path();
+  base::StringPiece url_path = url.path();
   if (url_path.empty() || url_path[0] != '/')
     return base::FilePath();
 
   // Drop the leading slashes and convert %-encoded UTF8 to regular UTF8.
   std::string file_path = net::UnescapeURLComponent(
-      url_path,
+      url_path.as_string(),
       net::UnescapeRule::SPACES |
           net::UnescapeRule::URL_SPECIAL_CHARS_EXCEPT_PATH_SEPARATORS);
   size_t skip = file_path.find_first_not_of("/\\");