Fix the x-offset calculation when drawing the text.

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 e4469a6fdfa2500f117317a0b22e84bfac6deb4c..cd03d6809c34a52c5e63fcec79618e3209885f79 100644
--- a/ui/gfx/render_text_harfbuzz.cc
+++ b/ui/gfx/render_text_harfbuzz.cc
@@ -960,8 +960,9 @@ void RenderTextHarfBuzz::DrawVisualText(Canvas* canvas) {
                                    background_is_transparent());
       Range glyphs_range = run.CharRangeToGlyphRange(segment.char_range);
       scoped_ptr<SkPoint[]> positions(new SkPoint[glyphs_range.length()]);
-      SkScalar offset_x =
-          preceding_segment_widths - run.positions[glyphs_range.start()].x();
+      SkScalar offset_x = preceding_segment_widths -

[msw, 2015/02/14 04:22:34]

Hmm, I wonder if this is actually correct, or if really should just be:
  SkScalar offset_x = preceding_segment_widths;
For single-line, there's one segment per run, so glyphs_range.start() is always
|0|, right? So, this is just adding run.positions[0].x() to the subtraction of
run.positions[0].x()... Can you debug and check if this is ever really untrue?

Otherwise, in the multi-line case, I could see why we'd subtract the position of
the first glyph in the range if the segment began mid-run (it didn't contain the
start of the run), because its run started on a previous line... In that case,
we'd want to keep the subtraction of run.positions[glyphs_range.start()].x(),
but not add the position of the first glyph... so maybe it's:
  SkScalar offset_x = preceding_segment_widths -
    (glyphs_range.start() > 0) ? run.positions[glyphs_range.start()].x() : 0;

Maybe Cem has a better idea of what's going on here...

[Jun Mukai, 2015/02/17 21:24:21]

Thanks, you are right. My intension was:
- if a segment starts from the beginning of a run, then, we should ignore the x
position because it's canceled below
- if a segment starts from the middle of a run (can happen in multi-line case),
then offset_x should consider the x-posistion from the beginning of the run,
this is essentially run.positions[glyphs_range.start()]

Done as you suggested.

+                          run.positions[glyphs_range.start()].x() +
+                          run.positions[0].x();
       for (size_t j = 0; j < glyphs_range.length(); ++j) {
         positions[j] = run.positions[(glyphs_range.is_reversed()) ?
                                      (glyphs_range.start() - j) :