For DirectWrite font metrics use the average character width from the TEXTMETRIC structure.

ui/gfx/platform_font_win.cc

 // Copyright (c) 2012 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.
 
 #include "ui/gfx/platform_font_win.h"
 
 #include <algorithm>
 #include <dwrite.h>
 #include <math.h>
 #include <windows.h>
@@ skipping 491 matching lines @@
   // function. The height, baseline and cap height are rounded up to ensure
   // that they match up closely with GDI.
   const int height = std::ceil(skia_metrics.fDescent - skia_metrics.fAscent);
   const int baseline = std::max<int>(1, std::ceil(-skia_metrics.fAscent));
   const int cap_height = std::ceil(paint.getTextSize() *
       static_cast<double>(dwrite_font_metrics.capHeight) /
           dwrite_font_metrics.designUnitsPerEm);
 
   // The metrics retrieved from skia don't have the average character width. In
   // any case if we get the average character width from skia then use that or
-  // use the text extent technique as documented by microsoft. See
-  // GetAverageCharWidthInDialogUnits for details.
+  // the average character width in the TEXTMETRIC structure.
+  // TODO(ananta)

[Alexei Svitkine (slow), 2015/01/29 20:52:10]

Nit:

Usually, there's a : after the TODO(name) and the text follows on the same line.

[ananta, 2015/01/29 21:04:58]

Done.

+  // Investigate whether it is possible to retrieve this value from
+  // DirectWrite.
   const int ave_char_width =
-      skia_metrics.fAvgCharWidth == 0 ?
-          HFontRef::GetAverageCharWidthInDialogUnits(gdi_font)
+      skia_metrics.fAvgCharWidth == 0 ? font_metrics.tmAveCharWidth
               : skia_metrics.fAvgCharWidth;
 
   int style = 0;
   if (skia_style & SkTypeface::kItalic)
     style |= Font::ITALIC;
   if (font_info.lfUnderline)
     style |= Font::UNDERLINE;
   if (font_info.lfWeight >= kTextMetricWeightBold)
     style |= Font::BOLD;
   return new HFontRef(gdi_font, -font_info.lfHeight, height, baseline,
@@ skipping 83 matching lines @@
   return new PlatformFontWin(native_font);
 }
 
 // static
 PlatformFont* PlatformFont::CreateFromNameAndSize(const std::string& font_name,
                                                   int font_size) {
   return new PlatformFontWin(font_name, font_size);
 }
 
 }  // namespace gfx