Pepper: Make PPB_FileChooser_Dev::Show "return" PP_ERROR_USERCANCEL if no file is selected.

ppapi/c/dev/ppb_file_chooser_dev.h

 /* Copyright (c) 2010 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.
  */
 #ifndef PPAPI_C_DEV_PPB_FILE_CHOOSER_DEV_H_
 #define PPAPI_C_DEV_PPB_FILE_CHOOSER_DEV_H_
 
 #include "ppapi/c/pp_bool.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_macros.h"
 #include "ppapi/c/pp_resource.h"
 
 struct PP_CompletionCallback;
 
 typedef enum {
   PP_FILECHOOSERMODE_OPEN,
   PP_FILECHOOSERMODE_OPENMULTIPLE
   // TODO(darin): Should there be a way to choose a directory?
 } PP_FileChooserMode_Dev;
 PP_COMPILE_ASSERT_SIZE_IN_BYTES(PP_FileChooserMode_Dev, 4);
 
 struct PP_FileChooserOptions_Dev {
   PP_FileChooserMode_Dev mode;
 
   // A comma-separated list of MIME types such as audio/*,text/plain.  The
-  // dialog may restrict selectable files to the specified MIME types.
+  // dialog may restrict selectable files to the specified MIME types. Null may
+  // be given to indicate that all types should be accepted.
   // TODO(darin): What if the mime type is unknown to the system?  The plugin
   // may wish to describe the mime type and provide a matching file extension.
   // It is more webby to use mime types here instead of file extensions.
   const char* accept_mime_types;
 };
 
 #define PPB_FILECHOOSER_DEV_INTERFACE "PPB_FileChooser(Dev);0.3"
 
 struct PPB_FileChooser_Dev {
   // Creates a file chooser dialog with the specified options.  The chooser is
   // associated with a particular instance, so that it may be positioned on the
   // screen relative to the tab containing the instance.  Returns 0 if passed
   // an invalid instance.
   //
   // A PPB_FileChooser_Dev instance can be used to select a single file
   // (PP_FILECHOOSERMODE_OPEN) or multiple files
   // (PP_FILECHOOSERMODE_OPENMULTIPLE). Unlike the HTML5 <input type="file">
   // tag, a PPB_FileChooser_Dev instance cannot be used to select a directory.
   // In order to get the list of files in a directory, the
   // PPB_DirectoryReader_Dev interface must be used.
   PP_Resource (*Create)(PP_Instance instance,
                         const struct PP_FileChooserOptions_Dev* options);
 
   // Returns PP_TRUE if the given resource is a FileChooser. Returns PP_FALSE
   // if the resource is invalid or some type other than a FileChooser.
   PP_Bool (*IsFileChooser)(PP_Resource resource);
 
-  // Prompts the user to choose a file or files.
+  // Prompts the user to choose a file or files. The callback is called with
+  // PP_OK on successful completion with a file (or files) selected or
+  // PP_ERROR_USERCANCEL if the user selected no file.
   int32_t (*Show)(PP_Resource chooser, struct PP_CompletionCallback callback);
 
   // After a successful call to Show, this method may be used to query the
   // chosen files.  It should be called in a loop until it returns 0.
   // Depending on the PP_ChooseFileMode requested when the FileChooser was
   // created, the file refs will either be readable or writable.  Their file
   // system type will be PP_FileSystemType_External.  If the user chose no
   // files or cancelled the dialog, then this method will simply return 0
   // the first time it is called.
   PP_Resource (*GetNextChosenFile)(PP_Resource chooser);
 };
 
 #endif  /* PPAPI_C_DEV_PPB_FILE_CHOOSER_DEV_H_ */
-