Make RenderObject::style() return a const object

Source/core/layout/LayoutSliderThumb.cpp

Index: Source/core/layout/LayoutSliderThumb.cpp
diff --git a/Source/core/layout/LayoutSliderThumb.cpp b/Source/core/layout/LayoutSliderThumb.cpp
index 462bd50ea115f0d97ae0726287ac4dc2d6060af9..d419e307159bf345f96155be4da068591b4a6bd1 100644
--- a/Source/core/layout/LayoutSliderThumb.cpp
+++ b/Source/core/layout/LayoutSliderThumb.cpp
@@ -45,15 +45,15 @@ LayoutSliderThumb::LayoutSliderThumb(SliderThumbElement* element)
 void LayoutSliderThumb::updateAppearance(const LayoutStyle& parentStyle)
 {
     if (parentStyle.appearance() == SliderVerticalPart)
-        style()->setAppearance(SliderThumbVerticalPart);
+        mutableStyleRef().setAppearance(SliderThumbVerticalPart);
     else if (parentStyle.appearance() == SliderHorizontalPart)
-        style()->setAppearance(SliderThumbHorizontalPart);
+        mutableStyleRef().setAppearance(SliderThumbHorizontalPart);
     else if (parentStyle.appearance() == MediaSliderPart)
-        style()->setAppearance(MediaSliderThumbPart);
+        mutableStyleRef().setAppearance(MediaSliderThumbPart);
     else if (parentStyle.appearance() == MediaVolumeSliderPart)
-        style()->setAppearance(MediaVolumeSliderThumbPart);
+        mutableStyleRef().setAppearance(MediaVolumeSliderThumbPart);
     else if (parentStyle.appearance() == MediaFullScreenVolumeSliderPart)
-        style()->setAppearance(MediaFullScreenVolumeSliderThumbPart);
+        mutableStyleRef().setAppearance(MediaFullScreenVolumeSliderThumbPart);
     if (style()->hasAppearance())
         LayoutTheme::theme().adjustSliderThumbSize(mutableStyleRef(), toElement(node()));
 }