Implement right-to-left text rendering in Pepper.

ppapi/api/dev/ppb_font_dev.idl

 /* Copyright (c) 2012 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.
  */
 
 /**
  * This file defines the <code>PPB_Font_Dev</code> interface.
  */
 label Chrome {
   M14 = 0.6
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 [assert_size(24)]
 struct PP_TextRun_Dev {
   /**
    * This var must either be a string or a null/undefined var (which will be
    * treated as a 0-length string).
    */
   PP_Var text;
 
   /**
    * Set to PP_TRUE if the text is right-to-left.
+   *
+   * When <code>override_direction</code> is false, the browser will perform
+   * the Unicode Bidirectional Algorithm (http://unicode.org/reports/tr9/) on
+   * the text. The value of the <code>rtl</code> flag specifies the
+   * direcionality of the surrounding environment. This means that Hebrew
+   * word will always display right to left, even if <code>rtl</code> is false.
+   *
+   * When <code>override_direction</code> is true, no autodetection will be done
+   * and <code>rtl</code> specifies the direction of the text.
+   *
+   * TODO(brettw) note that autodetection with rtl = true is currently
+   * unimplemented.
    */
   PP_Bool rtl;
 
   /**
    * Set to PP_TRUE to force the directionality of the text regardless of
-   * content
+   * content.
+   *
+   * If this flag is set, the browser will skip autodetection of the content
+   * and will display all text in the direction speficied by the
+   * <code>rtl</code> flag.
    */
   PP_Bool override_direction;
 };
 
 interface PPB_Font_Dev {
   /**
    * Returns a list of all available font families on the system. You can use
    * this list to decide whether to Create() a font.
    *
    * The return value will be a single string with null characters delimiting
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    * placed at the given position. This handles complex scripts such as Arabic,
    * where characters may be combined or replaced depending on context. Returns
    * -1 on error.
    */
   int32_t PixelOffsetForCharacter(
       [in] PP_Resource font,
       [in] PP_TextRun_Dev text,
       [in] uint32_t char_offset);
 };