mediaview: Implement ArcDocumentsProviderWatcherManager.

chrome/browser/chromeos/fileapi/file_system_backend.cc

Index: chrome/browser/chromeos/fileapi/file_system_backend.cc
diff --git a/chrome/browser/chromeos/fileapi/file_system_backend.cc b/chrome/browser/chromeos/fileapi/file_system_backend.cc
index 299f128be9228cea22b5d873eb00e100f84be9fe..3ed4d84e8661906ec8598a0586968ffc6eae6bac 100644
--- a/chrome/browser/chromeos/fileapi/file_system_backend.cc
+++ b/chrome/browser/chromeos/fileapi/file_system_backend.cc
@@ -7,6 +7,7 @@
 #include <stddef.h>
 
 #include <memory>
+#include <utility>
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ -301,6 +302,9 @@ storage::WatcherManager* FileSystemBackend::GetWatcherManager(
     return mtp_delegate_->GetWatcherManager(type);
   }
 
+  if (type == storage::kFileSystemTypeArcDocumentsProvider)
+    return arc_documents_provider_delegate_->GetWatcherManager(type);
+
   // TODO(mtomasz): Add support for other backends.
   return NULL;
 }