[fsp] Add an information that root entry name should be an empty string

Before, it was not well defined what's the name of a root entry.

TEST=None
BUG=446437

Review URL: https://codereview.chromium.org/881903002

Cr-Commit-Position: refs/heads/master@{#313379}

chrome/common/extensions/api/file_system_provider.idl

@@ -46,7 +46,8 @@ namespace fileSystemProvider {
     // True if it is a directory.
     boolean isDirectory;
 
-    // Name of this entry (not full path name). Must not contain '/'.
+    // Name of this entry (not full path name). Must not contain '/'. For root
+    // it must be empty.
     DOMString name;
 
     // File size in bytes.