Add a comment explaining a different failure mode of the Pepper file chooser.

ppapi/c/dev/ppb_file_chooser_dev.h

Index: ppapi/c/dev/ppb_file_chooser_dev.h
diff --git a/ppapi/c/dev/ppb_file_chooser_dev.h b/ppapi/c/dev/ppb_file_chooser_dev.h
index 469c712fad1b1e54f0d53a20ec11012649a96955..ad43a0fb9796d52ad2899ba5c58eeb40cb4ee2d3 100644
--- a/ppapi/c/dev/ppb_file_chooser_dev.h
+++ b/ppapi/c/dev/ppb_file_chooser_dev.h
@@ -3,7 +3,7 @@
  * found in the LICENSE file.
  */
 
-/* From dev/ppb_file_chooser_dev.idl modified Thu May 17 09:04:27 2012. */
+/* From dev/ppb_file_chooser_dev.idl modified Mon Jun  4 12:44:29 2012. */
 
 #ifndef PPAPI_C_DEV_PPB_FILE_CHOOSER_DEV_H_
 #define PPAPI_C_DEV_PPB_FILE_CHOOSER_DEV_H_
@@ -97,6 +97,16 @@ struct PPB_FileChooser_Dev_0_6 {
    * with a file (or files) selected, PP_ERROR_USERCANCEL if the user selected
    * no file, or another error code from pp_errors.h on failure.
    *
+   * <b>Subtle note:</b> This function will only work when the tab containing
+   * the plugin is visible. Show() will fail if the tab is in the background.
+   * Since it's not normally possible to get input events while invisible, this
+   * is not normally an issue. But there is a race condition because events are
+   * processed asynchronously. If the user clicks and switches tabs very
+   * quickly, a plugin could believe the tab is visible while Chrome believes
+   * it is invisible and the Show() call will fail. This will not generally
+   * cause user confusion since the user will have switched tabs and will not
+   * want to see a file chooser from a different tab.
+   *
    * @param[in] chooser The file chooser resource.
    *
    * @param[in] output An output array which will receive PP_Resource(s)