Rework FileWatcher to avoid race condition upon deletion.

chrome/browser/file_watcher.h

Index: chrome/browser/file_watcher.h
diff --git a/chrome/browser/file_watcher.h b/chrome/browser/file_watcher.h
index f24be2e8ac3b153be6adc41ab7685fed897d2d74..2d4417bcc00c839c93f28ef1fbdd9cc14ae5d454 100644
--- a/chrome/browser/file_watcher.h
+++ b/chrome/browser/file_watcher.h
@@ -25,7 +25,10 @@ class FilePath;
 // 2s of the file having changed.
 class FileWatcher {
  public:
-  class Delegate {
+  // Declares the callback client code implements to receive notifications. Note
+  // that implementations of this interface should not keep a reference to the
+  // corresponding FileWatcher object to prevent a reference cycle.
+  class Delegate : public base::RefCountedThreadSafe<Delegate> {
    public:
     virtual ~Delegate() {}
     virtual void OnFileChanged(const FilePath& path) = 0;