[base] Make FilePathWatcher::Watch virtual so that it can be mocked.

base/files/file_path_watcher.h

Index: base/files/file_path_watcher.h
diff --git a/base/files/file_path_watcher.h b/base/files/file_path_watcher.h
index 9c27ce4605cd28accba4a3912df54227cf3c3c6c..75416bc31d86f44d3fe53f27668e7088921d0d10 100644
--- a/base/files/file_path_watcher.h
+++ b/base/files/file_path_watcher.h
@@ -91,7 +91,7 @@ class BASE_EXPORT FilePathWatcher {
   };
 
   FilePathWatcher();
-  ~FilePathWatcher();
+  virtual ~FilePathWatcher();
 
   // A callback that always cleans up the PlatformDelegate, either when executed
   // or when deleted without having been executed at all, as can happen during
@@ -102,7 +102,8 @@ class BASE_EXPORT FilePathWatcher {
   // back for each change. Returns true on success.
   // OnFilePathChanged() will be called on the same thread as Watch() is called,
   // which should have a MessageLoop of TYPE_IO.
-  bool Watch(const FilePath& path, Delegate* delegate) WARN_UNUSED_RESULT;
+  virtual bool Watch(const FilePath& path, Delegate* delegate)
+      WARN_UNUSED_RESULT;
 
  private:
   scoped_refptr<PlatformDelegate> impl_;