Set root resource ID upon full feed update.

chrome/browser/chromeos/drive/drive_file_system.cc

Index: chrome/browser/chromeos/drive/drive_file_system.cc
diff --git a/chrome/browser/chromeos/drive/drive_file_system.cc b/chrome/browser/chromeos/drive/drive_file_system.cc
index 908001e88b68d23ffaab95aff5556303a75d19a6..0c35c357f9a3b462290165c2f89debcdcec5c8b8 100644
--- a/chrome/browser/chromeos/drive/drive_file_system.cc
+++ b/chrome/browser/chromeos/drive/drive_file_system.cc
@@ -42,6 +42,12 @@ using content::BrowserThread;
 namespace drive {
 namespace {
 
+// The resource ID for the root directory for WAPI is defined in the spec:
+// https://developers.google.com/google-apps/documents-list/
+// Note that this special ID only applies to WAPI. Drive uses a non-constant
+// unique ID given in About resource.
+const char kWAPIRootDirectoryResourceId[] = "folder:root";
+
 const char kMimeTypeJson[] = "application/json";
 const char kEmptyFilePath[] = "/dev/null";
 
@@ -392,6 +398,10 @@ void DriveFileSystem::Initialize() {
   drive_service_->Initialize(profile_);
 
   resource_metadata_.reset(new DriveResourceMetadata);
+  if (!google_apis::util::IsDriveV2ApiEnabled())
+    resource_metadata_->InitializeRootEntry(kWAPIRootDirectoryResourceId);
+  // If Drive API is enabled, the root directory is initialized after
+  // About resource is obtained.
   feed_loader_.reset(new DriveFeedLoader(resource_metadata_.get(),
                                          drive_service_,
                                          webapps_registry_,