mediaview: Implement ArcDocumentsProviderWatcherManager.

chrome/browser/chromeos/arc/fileapi/arc_documents_provider_backend_delegate.cc

Index: chrome/browser/chromeos/arc/fileapi/arc_documents_provider_backend_delegate.cc
diff --git a/chrome/browser/chromeos/arc/fileapi/arc_documents_provider_backend_delegate.cc b/chrome/browser/chromeos/arc/fileapi/arc_documents_provider_backend_delegate.cc
index 8e00c4e6753cb58025f255f05fa5b11e3bb9b273..10dd24cd615d345faec8f20c526f323982a8b0cc 100644
--- a/chrome/browser/chromeos/arc/fileapi/arc_documents_provider_backend_delegate.cc
+++ b/chrome/browser/chromeos/arc/fileapi/arc_documents_provider_backend_delegate.cc
@@ -19,7 +19,7 @@ using content::BrowserThread;
 namespace arc {
 
 ArcDocumentsProviderBackendDelegate::ArcDocumentsProviderBackendDelegate()
-    : async_file_util_(&roots_) {}
+    : async_file_util_(&roots_), watcher_manager_(&roots_) {}
 
 ArcDocumentsProviderBackendDelegate::~ArcDocumentsProviderBackendDelegate() {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
@@ -57,8 +57,7 @@ ArcDocumentsProviderBackendDelegate::CreateFileStreamWriter(
 storage::WatcherManager* ArcDocumentsProviderBackendDelegate::GetWatcherManager(
     storage::FileSystemType type) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
-  NOTREACHED();  // Non-watchable file system.
-  return nullptr;
+  return &watcher_manager_;
 }
 
 void ArcDocumentsProviderBackendDelegate::GetRedirectURLForContents(