Call WatcherManager functions on the IO thread.

storage/browser/fileapi/watcher_manager.h

Index: storage/browser/fileapi/watcher_manager.h
diff --git a/storage/browser/fileapi/watcher_manager.h b/storage/browser/fileapi/watcher_manager.h
index 64c310663e12002d2fb73d3770ae6fcd94ce2ed3..0d5901b8ac8d49695cb0ebec842c6d1b2ffb3f6e 100644
--- a/storage/browser/fileapi/watcher_manager.h
+++ b/storage/browser/fileapi/watcher_manager.h
@@ -17,6 +17,9 @@ class FileSystemURL;
 // An interface for providing entry observing capability for file system
 // backends.
 //
+// All member functions must be called on the IO thread. Callbacks will be
+// called on the IO thread.
+//
 // It is NOT valid to give null callback to this class, and implementors
 // can assume that they don't get any null callbacks.
 class WatcherManager {