Improve RenderTextWin font fallback.

ui/gfx/render_text.h

 // Copyright (c) 2011 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 UI_GFX_RENDER_TEXT_H_
 #define UI_GFX_RENDER_TEXT_H_
 #pragma once
 
 #include <algorithm>
 #include <vector>
@@ skipping 216 matching lines @@
   // render the text and its attributes.
   virtual void UpdateLayout() = 0;
 
   // Ensure the text is laid out.
   virtual void EnsureLayout() = 0;
 
   // Draw the text.
   virtual void DrawVisualText(Canvas* canvas) = 0;
 
   // Get the logical index of the grapheme preceding the argument |position|.
+  // If |IsCursorablePosition(position)| is true, the result will be the start
+  // of the previous grapheme, if any. Otherwise, the result will be the start
+  // of the grapheme containing |position|.
   size_t GetIndexOfPreviousGrapheme(size_t position);
 
   // Apply composition style (underline) to composition range and selection
   // style (foreground) to selection range.
   void ApplyCompositionAndSelectionStyles(StyleRanges* style_ranges) const;
 
   // Convert points from the text space to the view space and back.
   // Handles the display area, display offset, and the application LTR/RTL mode.
   Point ToTextPoint(const Point& point);
   Point ToViewPoint(const Point& point);
 
  private:
   friend class RenderTextTest;
 
   FRIEND_TEST_ALL_PREFIXES(RenderTextTest, DefaultStyle);
   FRIEND_TEST_ALL_PREFIXES(RenderTextTest, CustomDefaultStyle);
   FRIEND_TEST_ALL_PREFIXES(RenderTextTest, ApplyStyleRange);
   FRIEND_TEST_ALL_PREFIXES(RenderTextTest, StyleRangesAdjust);
+  FRIEND_TEST_ALL_PREFIXES(RenderTextTest, GraphemePositions);
+  FRIEND_TEST_ALL_PREFIXES(RenderTextTest, SelectionModels);
 
   // Return an index belonging to the |next| or previous logical grapheme.
+  // If |next| is false and |IsCursorablePosition(index)| is true, the result
+  // will be the start of the previous grapheme, if any. Otherwise, the result
+  // will be the start of the grapheme containing |index|.
   // The return value is bounded by 0 and the text length, inclusive.
   virtual size_t IndexOfAdjacentGrapheme(size_t index, bool next) = 0;
 
   // Set the cursor to |position|, with the caret trailing the previous
   // grapheme, or if there is no previous grapheme, leading the cursor position.
   // If |select| is false, the selection start is moved to the same position.
   // If the |position| is not a cursorable position (not on grapheme boundary),
   // it is a NO-OP.
   void MoveCursorTo(size_t position, bool select);
 
@@ skipping 38 matching lines @@
   // The cached bounds and offset are invalidated by changes to the cursor,
   // selection, font, and other operations that adjust the visible text bounds.
   bool cached_bounds_and_offset_valid_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderText);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_RENDER_TEXT_H_