[fsp] Add support for providing thumbnails.

chrome/common/extensions/api/file_system_provider.idl

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Use the <code>chrome.fileSystemProvider</code> API to create file systems,
 // that can be accessible from the file manager on Chrome OS.
 [platforms=("chromeos"),
  implemented_in="chrome/browser/chromeos/extensions/file_system_provider/file_system_provider_api.h"]
 namespace fileSystemProvider {
   // Error codes used by providing extensions in response to requests. For
@@ skipping 31 matching lines @@
     // Name of this entry (not full path name).
     DOMString name;
 
     // File size in bytes.
     double size;
 
     // The last modified time of this entry.
     [instanceOf=Date] object modificationTime;
 
     // Mime type for the entry.
-    DOMString? mimeType; 
+    DOMString? mimeType;
+
+    // Thumbnail image as a data URI in either PNG, JPEG or WEBP format, at most
+    // 32 KB in size. Optional, but can be provided only when explicitly
+    // requested by the <code>onGetMetadataRequested</code> event.
+    DOMString? thumbnail;
   };
 
   // Represents a mounted file system.
   dictionary FileSystemInfo {
     DOMString fileSystemId;
     DOMString displayName;
     [nodoc] boolean writable;
   };
 
   // Options for the <code>mount()</code> method.
@@ skipping 12 matching lines @@
   dictionary UnmountRequestedOptions {
     DOMString fileSystemId;
     long requestId;
   };
 
   // Options for the <code>onGetMetadataRequested()</code> event.
   dictionary GetMetadataRequestedOptions {
     DOMString fileSystemId;
     long requestId;
     DOMString entryPath;
+
+    // Set to <code>true</code> if the thumbnail is requested.
+    boolean thumbnail;
   };
 
   // Options for the <code>onReadDirectoryRequested()</code> event.
   dictionary ReadDirectoryRequestedOptions {
     DOMString fileSystemId;
     long requestId;
     DOMString directoryPath;
   };
 
   // Options for the <code>onOpenFileRequested()</code> event.
@@ skipping 260 matching lines @@
     // </code> must be called. Note, that callbacks of the aborted operation
     // should not be called, as they will be ignored. Despite calling <code>
     // errorCallback</code>, the request may be forcibly aborted. 
     [maxListeners=1, nodoc] static void onAbortRequested(
         AbortRequestedOptions options,
         ProviderSuccessCallback successCallback,
         ProviderErrorCallback errorCallback);
   };
 };