Require FilePathWatcher destructor to be called in sequence with Watch().

base/files/file_path_watcher.cc

Index: base/files/file_path_watcher.cc
diff --git a/base/files/file_path_watcher.cc b/base/files/file_path_watcher.cc
index e370fcb1355f0345238019cad8060b20f10b409f..245bd8efe22d3106602140aa8a3aeb5487cfbbf6 100644
--- a/base/files/file_path_watcher.cc
+++ b/base/files/file_path_watcher.cc
@@ -8,12 +8,12 @@
 #include "base/files/file_path_watcher.h"
 
 #include "base/logging.h"
-#include "base/message_loop/message_loop.h"
 #include "build/build_config.h"
 
 namespace base {
 
 FilePathWatcher::~FilePathWatcher() {
+  DCHECK(sequence_checker_.CalledOnValidSequence());
   impl_->Cancel();
 }
 
@@ -38,6 +38,7 @@ FilePathWatcher::PlatformDelegate::~PlatformDelegate() {
 bool FilePathWatcher::Watch(const FilePath& path,
                             bool recursive,
                             const Callback& callback) {
+  DCHECK(sequence_checker_.CalledOnValidSequence());
   DCHECK(path.IsAbsolute());
   return impl_->Watch(path, recursive, callback);
 }