linux: Use subpixel positioning for forced scale factor.

ui/gfx/font_render_params_linux.cc

Index: ui/gfx/font_render_params_linux.cc
diff --git a/ui/gfx/font_render_params_linux.cc b/ui/gfx/font_render_params_linux.cc
index 5e30f4dd947acf73a05e435d2dfe8420bdfd0aeb..0c157d578f2e6c74fc9a48b1a9feb3268ebe95c5 100644
--- a/ui/gfx/font_render_params_linux.cc
+++ b/ui/gfx/font_render_params_linux.cc
@@ -6,6 +6,7 @@
 
 #include "base/command_line.h"
 #include "base/logging.h"
+#include "ui/gfx/display.h"
 #include "ui/gfx/switches.h"
 
 #if defined(TOOLKIT_GTK)
@@ -23,10 +24,16 @@ 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