Revert of arc: Fix thread unsafe behavior of ArcDownloadsWatcherService

chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc

Index: chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc
diff --git a/chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc b/chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc
index ea465221e2c9a7b9b445ebfbda1b7a0342367acd..8bae833b113d0fc53cc32e5984c8812d47347b3b 100644
--- a/chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc
+++ b/chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc
@@ -76,7 +76,8 @@
 // Downloads directory.
 class ArcDownloadsWatcherService::DownloadsWatcher {
  public:
-  using Callback = base::Callback<void(mojo::Array<mojo::String> paths)>;
+  using Callback =
+      base::Callback<void(const std::vector<base::FilePath>& paths)>;
 
   explicit DownloadsWatcher(const Callback& callback);
   ~DownloadsWatcher();
@@ -146,13 +147,7 @@
 
   last_timestamp_map_ = std::move(current_timestamp_map);
 
-  mojo::Array<mojo::String> mojo_paths(changed_paths.size());
-  for (size_t i = 0; i < changed_paths.size(); ++i) {
-    mojo_paths[i] = changed_paths[i].value();
-  }
-  BrowserThread::PostTask(
-      BrowserThread::UI, FROM_HERE,
-      base::Bind(callback_, base::Passed(std::move(mojo_paths))));
+  callback_.Run(changed_paths);
 }
 
 TimestampMap ArcDownloadsWatcherService::DownloadsWatcher::BuildTimestampMap()
@@ -222,13 +217,19 @@
 }
 
 void ArcDownloadsWatcherService::OnDownloadsChanged(
-    mojo::Array<mojo::String> paths) {
-  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+    const std::vector<base::FilePath>& paths) {
+  DCHECK_CURRENTLY_ON(BrowserThread::FILE);
 
   auto instance = arc_bridge_service()->file_system()->instance();
   if (!instance) {
     return;
-  instance->RequestMediaScan(std::move(paths));
+  }
+
+  mojo::Array<mojo::String> mojo_paths(paths.size());
+  for (size_t i = 0; i < paths.size(); ++i) {
+    mojo_paths[i] = paths[i].value();
+  }
+  instance->RequestMediaScan(std::move(mojo_paths));
 }
 
 }  // namespace arc