Add DCHECK to address a comment in CL:1548974

BUG=921357
TEST=try

Change-Id: I05b426832e5de4d1a2d5d340b408fdc06e9b97ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1579560
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653377}

base/files/file_path_watcher_linux.cc

@@ -516,6 +516,8 @@ bool FilePathWatcherImpl::IncreaseWatch() {
 
 void FilePathWatcherImpl::DecreaseWatch() {
   --number_of_inotify_watches_;
+  // Sanity check:
+  DCHECK_GE(number_of_inotify_watches_, 0);
 }
 
 bool FilePathWatcherImpl::Watch(const FilePath& path,