Factor painting code from RenderLayerScrollableArea into ScrollableAreaPainter

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/AnimationNode.idl',
@@ skipping 1497 matching lines @@
             'paint/SVGInlineTextBoxPainter.cpp',
             'paint/SVGInlineTextBoxPainter.h',
             'paint/SVGRootInlineBoxPainter.cpp',
             'paint/SVGRootInlineBoxPainter.h',
             'paint/SVGRootPainter.cpp',
             'paint/SVGRootPainter.h',
             'paint/SVGShapePainter.cpp',
             'paint/SVGShapePainter.h',
             'paint/SVGTextPainter.cpp',
             'paint/SVGTextPainter.h',
+            'paint/ScrollableAreaPainter.cpp',
+            'paint/ScrollableAreaPainter.h',
             'paint/ScrollbarPainter.cpp',
             'paint/ScrollbarPainter.h',
             'paint/TableCellPainter.cpp',
             'paint/TableCellPainter.h',
             'paint/TablePainter.cpp',
             'paint/TablePainter.h',
             'paint/TableRowPainter.cpp',
             'paint/TableRowPainter.h',
             'paint/TableSectionPainter.cpp',
             'paint/TableSectionPainter.h',
@@ skipping 2067 matching lines @@
             'rendering/style/OutlineValueTest.cpp',
             'rendering/style/RenderStyleTest.cpp',
             'streams/ReadableStreamTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }