[fsp] Add support for providing thumbnails.

chrome/browser/chromeos/file_system_provider/provided_file_system_interface.h

Index: chrome/browser/chromeos/file_system_provider/provided_file_system_interface.h
diff --git a/chrome/browser/chromeos/file_system_provider/provided_file_system_interface.h b/chrome/browser/chromeos/file_system_provider/provided_file_system_interface.h
index 064276fc449b5fba27ea4265967254c8e423dc38..740ad119a68018d47ed7838a2beff82a63b88e94 100644
--- a/chrome/browser/chromeos/file_system_provider/provided_file_system_interface.h
+++ b/chrome/browser/chromeos/file_system_provider/provided_file_system_interface.h
@@ -40,6 +40,7 @@ struct EntryMetadata {
   int64 size;
   base::Time modification_time;
   std::string mime_type;
+  std::string thumbnail;
 };
 
 // Interface for a provided file system. Acts as a proxy between providers
@@ -49,6 +50,15 @@ struct EntryMetadata {
 // fails synchronously.
 class ProvidedFileSystemInterface {
  public:
+  // Mode of opening a file. Used by OpenFile().
+  enum OpenFileMode { OPEN_FILE_MODE_READ, OPEN_FILE_MODE_WRITE };
+
+  // Extra fields to be fetched with metadata.
+  enum MetadataField {
+    METADATA_FIELD_DEFAULT = 0,
+    METADATA_FIELD_THUMBNAIL = 1 << 0
+  };
+
   typedef base::Callback<void(int file_handle, base::File::Error result)>
       OpenFileCallback;
 
@@ -62,8 +72,8 @@ class ProvidedFileSystemInterface {
   typedef base::Callback<void(
       const storage::AsyncFileUtil::StatusCallback& callback)> AbortCallback;
 
-  // Mode of opening a file. Used by OpenFile().
-  enum OpenFileMode { OPEN_FILE_MODE_READ, OPEN_FILE_MODE_WRITE };
+  // Mask of fields requested from the GetMetadata() call.
+  typedef int MetadataFieldMask;
 
   virtual ~ProvidedFileSystemInterface() {}
 
@@ -73,8 +83,10 @@ class ProvidedFileSystemInterface {
       const storage::AsyncFileUtil::StatusCallback& callback) = 0;
 
   // Requests metadata of the passed |entry_path|. It can be either a file
-  // or a directory.
+  // or a directory. All |fields| will be returned if supported. Note, that
+  // default fields are always returned.
   virtual AbortCallback GetMetadata(const base::FilePath& entry_path,
+                                    MetadataFieldMask fields,
                                     const GetMetadataCallback& callback) = 0;
 
   // Requests enumerating entries from the passed |directory_path|. The callback