Remove platform dependency upon page/Settings class which is a layering violation.

Source/core/page/Settings.cpp

Index: Source/core/page/Settings.cpp
diff --git a/Source/core/page/Settings.cpp b/Source/core/page/Settings.cpp
index ad1995bd4184fe9346ae62f404bde949458d855a..f3c2aaad3ab8f39d0916dba228a9514783f13fce 100644
--- a/Source/core/page/Settings.cpp
+++ b/Source/core/page/Settings.cpp
@@ -35,6 +35,7 @@
 #include "core/page/FrameTree.h"
 #include "core/page/FrameView.h"
 #include "core/page/Page.h"
+#include "core/platform/ScrollbarTheme.h"
 #include "core/rendering/TextAutosizer.h"
 
 using namespace std;
@@ -76,8 +77,6 @@ static inline const AtomicString& getGenericFontFamilyForScript(const ScriptFont
     return emptyAtom;
 }
 
-bool Settings::gMockScrollbarsEnabled = false;
-
 // NOTEs
 //  1) EditingMacBehavior comprises builds on Mac;
 //  2) EditingWindowsBehavior comprises builds on Windows;
@@ -346,12 +345,12 @@ void Settings::setDNSPrefetchingEnabled(bool dnsPrefetchingEnabled)
 
 void Settings::setMockScrollbarsEnabled(bool flag)
 {
-    gMockScrollbarsEnabled = flag;
+    ScrollbarTheme::setMockScrollbarsEnabled(flag);
 }
 
 bool Settings::mockScrollbarsEnabled()
 {
-    return gMockScrollbarsEnabled;
+    return ScrollbarTheme::mockScrollbarsEnabled();
 }
 
 void Settings::setOpenGLMultisamplingEnabled(bool flag)