Enable experimental support for sub-pixel font scaling

Source/core/platform/graphics/harfbuzz/FontPlatformDataHarfBuzz.cpp

 /*
  * Copyright (c) 2006, 2007, 2008, 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.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 172 matching lines @@
 }
 #endif
 
 void FontPlatformData::setupPaint(SkPaint* paint, GraphicsContext* context) const
 {
     UNUSED_PARAM(context);
     paint->setAntiAlias(m_style.useAntiAlias);
     paint->setHinting(static_cast<SkPaint::Hinting>(m_style.hintStyle));
     paint->setEmbeddedBitmapText(m_style.useBitmaps);
     paint->setAutohinted(m_style.useAutoHint);
-    paint->setSubpixelText(m_style.useSubpixelPositioning);
     if (m_style.useAntiAlias)
         paint->setLCDRenderText(m_style.useSubpixelRendering);
 
+    // TestRunner specifically toggles the subpixel positioning flag.
+    if (RuntimeEnabledFeatures::subpixelFontScalingEnabled() && !isRunningLayoutTest())
+        paint->setSubpixelText(true);
+    else
+        paint->setSubpixelText(m_style.useSubpixelPositioning);
+
     const float ts = m_textSize >= 0 ? m_textSize : 12;
     paint->setTextSize(SkFloatToScalar(ts));
     paint->setTypeface(m_typeface.get());
     paint->setFakeBoldText(m_fakeBold);
     paint->setTextSkewX(m_fakeItalic ? -SK_Scalar1 / 4 : 0);
 }
 
 SkFontID FontPlatformData::uniqueID() const
 {
     return m_typeface->uniqueID();
@@ skipping 81 matching lines @@
     if (m_style.useSubpixelRendering == FontRenderStyle::NoPreference)
         m_style.useSubpixelRendering = useSkiaSubpixelRendering;
 
     // TestRunner specifically toggles the subpixel positioning flag.
     if (m_style.useSubpixelPositioning == FontRenderStyle::NoPreference
         || isRunningLayoutTest())
         m_style.useSubpixelPositioning = useSkiaSubpixelPositioning;
 }
 
 } // namespace WebCore