Reject filesystem root entries as args for chrome.fileSystem functions.

chrome/browser/extensions/api/file_handlers/app_file_handler_util.cc

Index: chrome/browser/extensions/api/file_handlers/app_file_handler_util.cc
diff --git a/chrome/browser/extensions/api/file_handlers/app_file_handler_util.cc b/chrome/browser/extensions/api/file_handlers/app_file_handler_util.cc
index d8e3480fe2405ae7c58514167792f499394b8578..94a172ecaf1fba1eadcb873c209c813c3eb5349a 100644
--- a/chrome/browser/extensions/api/file_handlers/app_file_handler_util.cc
+++ b/chrome/browser/extensions/api/file_handlers/app_file_handler_util.cc
@@ -350,6 +350,11 @@ bool ValidateFileEntryAndGetPath(
     const content::RenderViewHost* render_view_host,
     base::FilePath* file_path,
     std::string* error) {
+  if (filesystem_path.empty()) {
+    *error = kInvalidParameters;
+    return false;
+  }
+
   std::string filesystem_id;
   if (!fileapi::CrackIsolatedFileSystemName(filesystem_name, &filesystem_id)) {
     *error = kInvalidParameters;