Avoid memory allocation for PPB_FileIO Read when callback is blocking.

ppapi/api/pp_array_output.idl

Index: ppapi/api/pp_array_output.idl
diff --git a/ppapi/api/pp_array_output.idl b/ppapi/api/pp_array_output.idl
index be1aca7bf8328064a7eb68e3235fdc7db9adb216..64c67bd5c52a6f0df74061638663cb83f33078db 100644
--- a/ppapi/api/pp_array_output.idl
+++ b/ppapi/api/pp_array_output.idl
@@ -9,7 +9,7 @@
  *
  * This function will be called reentrantly. This means that if you call a
  * function PPB_Foo.GetData(&array_output), GetData will call your
- * GetDataBuffer function before it returns. 
+ * GetDataBuffer function before it returns.
  *
  * This function will be called even when returning 0-length arrays, so be sure
  * your implementation can support that. You can return NULL for 0 length
@@ -34,7 +34,10 @@
  * @param element_size The size of each element in bytes.
  *
  * @return Returns a pointer to the allocated memory. On failure, returns null.
- * You can also return null if the element_count is 0.
+ * You can also return null if the element_count is 0. If memory is allocated on
+ * the heap, the plugin is responsible for freeing it after the callback runs.
+ * If used with a blocking callback, the memory must remain valid until the
+ * callback runs even if the plugin causes the operation to abort.
  */
 typedef mem_t PP_ArrayOutput_GetDataBuffer([inout] mem_t user_data,
                                            [in] uint32_t element_count,