Cleanup return values in chrome/common/extensions

This CL turns const by value return values into const by ref.

Replacing const value by a const ref when a member variable is returned
is safe, because the member variable will live at least as long as the
temporary object returned now. It can also be more efficient, because
returning a ref can spare copying.

BUG=393155

Review URL: https://codereview.chromium.org/423483006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285691 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/file_browser_handlers/file_browser_handler.h

@@ -48,7 +48,7 @@ class FileBrowserHandler {
   void ClearPatterns();
 
   // Action icon path.
-  const std::string icon_path() const { return default_icon_path_; }
+  const std::string& icon_path() const { return default_icon_path_; }
   void set_icon_path(const std::string& path) {
     default_icon_path_ = path;
   }

chrome/common/extensions/manifest_handlers/mime_types_handler.h

@@ -39,7 +39,7 @@ class MimeTypesHandler {
     handler_url_ = handler_url;
   }
   // The URL that will be used to handle MIME type requests.
-  const std::string handler_url() const { return handler_url_; }
+  const std::string& handler_url() const { return handler_url_; }
 
   const std::set<std::string>& mime_type_set() const { return mime_type_set_; }