Move paint code for RenderScrollbar and RenderScrollbarPart into ScrollbarPainter.

Source/core/core.gypi

 {
     'includes': [
       'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationEffect.idl',
             'animation/AnimationPlayer.idl',
@@ skipping 1502 matching lines @@
             'paint/ListMarkerPainter.cpp',
             'paint/ListMarkerPainter.h',
             'paint/ListItemPainter.cpp',
             'paint/ListItemPainter.h',
             'paint/ObjectPainter.cpp',
             'paint/ObjectPainter.h',
             'paint/MultiColumnSetPainter.cpp',
             'paint/MultiColumnSetPainter.h',
             'paint/ReplicaPainter.cpp',
             'paint/ReplicaPainter.h',
+            'paint/ScrollbarPainter.cpp',
+            'paint/ScrollbarPainter.h',
             'paint/SVGForeignObjectPainter.cpp',
             'paint/SVGForeignObjectPainter.h',
             'paint/SVGImagePainter.cpp',
             'paint/SVGImagePainter.h',
             'paint/SVGTextPainter.cpp',
             'paint/SVGTextPainter.h',
             'paint/TablePainter.cpp',
             'paint/TablePainter.h',
             'paint/TableRowPainter.cpp',
             'paint/TableRowPainter.h',
@@ skipping 2042 matching lines @@
             'rendering/shapes/BoxShapeTest.cpp',
             'rendering/style/OutlineValueTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'streams/ReadableStreamTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }