Improve RenderTextWin font fallback.

ui/gfx/render_text.h

Index: ui/gfx/render_text.h
===================================================================
--- ui/gfx/render_text.h	(revision 113627)
+++ ui/gfx/render_text.h	(working copy)
@@ -234,6 +234,9 @@
   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
@@ -252,8 +255,13 @@
   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;