Implement Pango RenderText for Linux.

ui/gfx/render_text.h

Index: ui/gfx/render_text.h
===================================================================
--- ui/gfx/render_text.h	(revision 97029)
+++ ui/gfx/render_text.h	(working copy)
@@ -121,9 +121,12 @@
  private:
   void Init(size_t start, size_t end, size_t pos, CaretPlacement status);
 
-  // Logical selection start. If there is non-empty selection, the selection
-  // always starts visually at the leading edge of the selection_start. So, we
-  // do not need extra information for visual selection bounding.
+  // Logical selection start. If there is non-empty selection, if
+  // selection_start_ is less than selection_end_, the selection starts visually
+  // at the leading edge of the selection_start_. If selection_start_ is greater
+  // than selection_end_, the selection starts visually at the trailing edge of
+  // selection_start_'s previous grapheme. So, we do not need extra information
+  // for visual bounding.
   size_t selection_start_;
 
   // The logical cursor position that next character will be inserted into.
@@ -298,6 +301,10 @@
   // cursor is within the visible display area.
   void UpdateCachedBoundsAndOffset();
 
+  // Returns the selection model of selection_start_.
+  // The returned value represents a cursor/caret position without a selection.
+  SelectionModel GetSelectionModelForSelectionStart();
+
   // Logical UTF-16 string data to be drawn.
   string16 text_;