Break runs by clusters rather than iteration over code points

ui/gfx/render_text_harfbuzz.cc

Index: ui/gfx/render_text_harfbuzz.cc
diff --git a/ui/gfx/render_text_harfbuzz.cc b/ui/gfx/render_text_harfbuzz.cc
index 3c71cff58c57c1b5dabf7aacff9bf7cc9764be6e..f3dd136fcc8431e0ac4297c3416be753d5c789fa 100644
--- a/ui/gfx/render_text_harfbuzz.cc
+++ b/ui/gfx/render_text_harfbuzz.cc
@@ -5,6 +5,7 @@
 #include "ui/gfx/render_text_harfbuzz.h"
 
 #include <limits>
+#include <set>
 
 #include "base/i18n/bidi_line_iterator.h"
 #include "base/i18n/break_iterator.h"
@@ -314,6 +315,7 @@ class HarfBuzzLineBreaker {
     // Width from |std::max(word->first, start_char)| to the current character.
     SkScalar word_width = 0;
     *width = 0;
+    std::set<size_t> used_glyphs;
 
     for (size_t i = start_char; i < run.range.end(); ++i) {
       // |word| holds the word boundary at or before |i|, and |next_word| holds
@@ -325,6 +327,19 @@ class HarfBuzzLineBreaker {
       }
 
       Range glyph_range = run.CharRangeToGlyphRange(Range(i, i + 1));
+
+      // Sometimes multiple code points can point to the same glyph (e.g.

[msw, 2015/03/24 22:52:45]

This fix seems odd/wrong. Shouldn't the line above just get the glyph range from
|i| to IndexOfAdjacentGrapheme(i, CURSOR_FORWARD)?

[Jun Mukai, 2015/03/25 00:23:15]

This is inside of EnsureLayout(), which is called from
IndexOfAdjacentGrapheme(), so cannot invoke that method.
Also, as far as I tested, IsGraphemeBoundary() returns true on this specific
example (see the new test case).

Considering about that, however, I think the right way is to use GetClusterAt()
rather than CharRangeToGlyphRange() and run over the clusters. PTAL.

+      // diacritic marks). We need to skip in that case to avoid adding
+      // the width of the same glyph multiple times.
+      bool already_used = false;
+      for (size_t i = glyph_range.start(); i < glyph_range.end(); ++i) {
+        if (used_glyphs.find(i) != used_glyphs.end())
+          already_used = true;
+        used_glyphs.insert(i);
+      }
+      if (already_used)
+        continue;
+
       SkScalar char_width = ((glyph_range.end() >= run.glyph_count)
                                  ? SkFloatToScalar(run.width)
                                  : run.positions[glyph_range.end()].x()) -