Prefer to forward-declare GraphicsContext in headers.

Source/platform/scroll/ScrollbarThemeMacCommon.mm

Index: Source/platform/scroll/ScrollbarThemeMacCommon.mm
diff --git a/Source/platform/scroll/ScrollbarThemeMacCommon.mm b/Source/platform/scroll/ScrollbarThemeMacCommon.mm
index 053088425633f711a4f8388ce569d41195d0692f..f3fdcf41e8499a9ba69c8f8d13c6f7631add5a75 100644
--- a/Source/platform/scroll/ScrollbarThemeMacCommon.mm
+++ b/Source/platform/scroll/ScrollbarThemeMacCommon.mm
@@ -34,6 +34,7 @@
 #include "platform/graphics/GraphicsContextStateSaver.h"
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/graphics/ImageBuffer.h"
+#include "platform/graphics/Pattern.h"
 #include "platform/mac/ColorMac.h"
 #include "platform/mac/LocalCurrentGraphicsContext.h"
 #include "platform/mac/NSScrollerImpDetails.h"
@@ -68,10 +69,6 @@ static ScrollbarSet& scrollbarSet()
     return set;
 }
 
-}
-
-namespace WebCore {
-
 static float gInitialButtonDelay = 0.5f;
 static float gAutoscrollButtonDelay = 0.05f;
 static NSScrollerStyle gPreferredScrollerStyle = NSScrollerStyleLegacy;