linux: Use subpixel positioning for forced scale factor.

ui/gfx/font_render_params_linux.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/font_render_params_linux.h"
 
 #include "base/command_line.h"
 #include "base/logging.h"
+#include "ui/gfx/display.h"
 #include "ui/gfx/switches.h"
 
 #if defined(TOOLKIT_GTK)
 #include <gtk/gtk.h>
 #else
 #include <fontconfig/fontconfig.h>
 #endif
 
 #if defined(OS_LINUX) && defined(USE_AURA) && !defined(OS_CHROMEOS)
 #include "ui/gfx/linux_font_delegate.h"
 #endif
 
 namespace gfx {
 
 namespace {
 
 bool SubpixelPositioningRequested(bool renderer) {
-  return CommandLine::ForCurrentProcess()->HasSwitch(
-      renderer ?
-      switches::kEnableWebkitTextSubpixelPositioning :
-      switches::kEnableBrowserTextSubpixelPositioning);
+  const CommandLine* cl = CommandLine::ForCurrentProcess();
+  if (renderer) {
+    // Text rendered by Blink in high-DPI mode is poorly-hinted unless subpixel
+    // positioning is used (as opposed to each glyph being individually snapped
+    // to the pixel grid).
+    return cl->HasSwitch(switches::kEnableWebkitTextSubpixelPositioning) ||
+           (Display::HasForceDeviceScaleFactor() &&
+            Display::GetForcedDeviceScaleFactor() != 1.0);
+  }
+  return cl->HasSwitch(switches::kEnableBrowserTextSubpixelPositioning);
 }
 
 // Initializes |params| with the system's default settings. |renderer| is true
 // when setting WebKit renderer defaults.
 void LoadDefaults(FontRenderParams* params, bool renderer) {
 #if defined(TOOLKIT_GTK)
   params->antialiasing = true;
   // TODO(wangxianzhu): autohinter is now true to keep original behavior
   // of WebKit, but it might not be the best value.
   params->autohinter = true;
@@ skipping 114 matching lines @@
     LoadDefaults(&default_params, /* renderer */ true);
   loaded_defaults = true;
   return default_params;
 }
 
 bool GetDefaultWebkitSubpixelPositioning() {
   return SubpixelPositioningRequested(true);
 }
 
 }  // namespace gfx