Avoid memory allocation for PPB_FileIO Read when callback is blocking.

ppapi/api/ppb_file_io.idl

Index: ppapi/api/ppb_file_io.idl
diff --git a/ppapi/api/ppb_file_io.idl b/ppapi/api/ppb_file_io.idl
index 2e43ec54c1c7ca7d7ffacbe3b6fbb17ac10a3008..9b8c9e22bdfd2c5b66fc335abe190c0fe2a721bf 100644
--- a/ppapi/api/ppb_file_io.idl
+++ b/ppapi/api/ppb_file_io.idl
@@ -121,7 +121,9 @@ interface PPB_FileIO {
    * @param[out] info The <code>PP_FileInfo</code> structure representing all
    * information about the file.
    * @param[in] callback A <code>PP_CompletionCallback</code> to be called upon
-   * completion of Query().
+   * completion of Query(). <code>info</code> must remain valid until after the
+   * callback runs. If you pass a blocking callback, <code>info</code> must
+   * remain valid until after Query() returns.
    *
    * @return An int32_t containing an error code from <code>pp_errors.h</code>.
    * PP_ERROR_FAILED will be returned if the file isn't opened, and
@@ -158,6 +160,7 @@ interface PPB_FileIO {
    * large enough to hold the specified number of bytes to read.  This function
    * might perform a partial read, meaning all the requested bytes
    * might not be returned, even if the end of the file has not been reached.
+   * The FileIO object must have been opened with read access.
    *
    * ReadToArray() is preferred to Read() when doing asynchronous operations.
    *
@@ -168,7 +171,9 @@ interface PPB_FileIO {
    * @param[in] bytes_to_read The number of bytes to read from
    * <code>offset</code>.
    * @param[in] callback A <code>PP_CompletionCallback</code> to be called upon
-   * completion of Read().
+   * completion of Read(). <code>buffer</code> must remain valid until after
+   * the callback runs. If you pass a blocking callback, <code>buffer</code>
+   * must remain valid until after Read() returns.
    *
    * @return The number of bytes read or an error code from
    * <code>pp_errors.h</code>. If the return value is 0, then end-of-file was
@@ -260,7 +265,8 @@ interface PPB_FileIO {
   /**
    * ReadToArray() reads from an offset in the file.  A PP_ArrayOutput must be
    * provided so that output will be stored in its allocated buffer.  This
-   * function might perform a partial read.
+   * function might perform a partial read. The FileIO object must have been
+   * opened with read access.
    *
    * @param[in] file_io A <code>PP_Resource</code> corresponding to a file
    * FileIO.