Enable experimental support for sub-pixel font scaling

Source/core/platform/graphics/win/FontPlatformDataWin.cpp

 /*
  * Copyright (C) 2006, 2007 Apple Computer, Inc.
  * Copyright (c) 2006, 2007, 2008, 2009, 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 40 matching lines @@
 
 namespace WebCore {
 
 void FontPlatformData::setupPaint(SkPaint* paint, GraphicsContext* context) const
 {
     const float ts = m_textSize >= 0 ? m_textSize : 12;
     paint->setTextSize(SkFloatToScalar(m_textSize));
     paint->setTypeface(typeface());
     paint->setFakeBoldText(m_fakeBold);
     paint->setTextSkewX(m_fakeItalic ? -SK_Scalar1 / 4 : 0);
-    if (RuntimeEnabledFeatures::subpixelFontScalingEnabled())
+
+    // Subpixel text positioning is not supported by the GDI backend.
+    if (RuntimeEnabledFeatures::directWriteEnabled()
+        && RuntimeEnabledFeatures::subpixelFontScalingEnabled())
         paint->setSubpixelText(true);
 
     int textFlags = paintTextFlags();
     // Only set painting flags when we're actually painting.
     if (context && !context->couldUseLCDRenderedText()) {
         textFlags &= ~SkPaint::kLCDRenderText_Flag;
         // If we *just* clear our request for LCD, then GDI seems to
         // sometimes give us AA text, and sometimes give us BW text. Since the
         // original intent was LCD, we want to force AA (rather than BW), so we
         // add a special bit to tell Skia to do its best to avoid the BW: by
@@ skipping 351 matching lines @@
 #endif
 
 #ifndef NDEBUG
 String FontPlatformData::description() const
 {
     return String();
 }
 #endif
 
 }