drive: Add the local largest changestamp to chrome:drive-internals

chrome/browser/chromeos/drive/drive_file_system_interface.h

Index: chrome/browser/chromeos/drive/drive_file_system_interface.h
diff --git a/chrome/browser/chromeos/drive/drive_file_system_interface.h b/chrome/browser/chromeos/drive/drive_file_system_interface.h
index 1f26b3747b1af459d9841a14f6ac631b46cc09db..a38c80f3c002eda101de15103a1206973fd942d1 100644
--- a/chrome/browser/chromeos/drive/drive_file_system_interface.h
+++ b/chrome/browser/chromeos/drive/drive_file_system_interface.h
@@ -21,6 +21,7 @@ namespace drive {
 
 class DriveEntryProto;
 class DriveFileSystemObserver;
+class DriveResourceMetadata;
 
 typedef std::vector<DriveEntryProto> DriveEntryProtoVector;
 
@@ -37,6 +38,14 @@ struct SearchResultInfo {
   bool is_directory;
 };
 
+// Metadata of DriveFileSystem. Used by DriveFileSystem::GetMetadata().
+struct DriveFileSystemMetadata {
+  DriveFileSystemMetadata() : largest_changestamp(0) {}
+  ~DriveFileSystemMetadata() {}
+
+  int64 largest_changestamp;
+};
+
 // Used to get files from the file system.
 typedef base::Callback<void(DriveFileError error,
                             const FilePath& file_path,
@@ -346,6 +355,9 @@ class DriveFileSystemInterface {
                                scoped_ptr<gdata::DocumentEntry> entry,
                                const FilePath& file_content_path,
                                const base::Closure& callback) = 0;
+
+  // Returns metadata of the file system.
+  virtual DriveFileSystemMetadata GetMetadata() const = 0;
 };
 
 }  // namespace drive