Implement lazy block layout prototype behind a runtime flag.

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSMediaRule.idl',
             'css/CSSPageRule.idl',
@@ skipping 1322 matching lines @@
             'rendering/RenderInline.h',
             'rendering/RenderInputSpeech.cpp',
             'rendering/RenderInputSpeech.h',
             'rendering/RenderLayer.cpp',
             'rendering/RenderLayerBacking.cpp',
             'rendering/RenderLayerCompositor.cpp',
             'rendering/RenderLayerCompositor.h',
             'rendering/RenderLayerFilterInfo.cpp',
             'rendering/RenderLayerFilterInfo.h',
             'rendering/RenderLayerModelObject.cpp',
+            'rendering/RenderLazyBlock.cpp',
+            'rendering/RenderLazyBlock.h',
             'rendering/RenderLineBoxList.cpp',
             'rendering/RenderListBox.cpp',
             'rendering/RenderListBox.h',
             'rendering/RenderListItem.cpp',
             'rendering/RenderListMarker.cpp',
             'rendering/RenderListMarker.h',
             'rendering/RenderMarquee.cpp',
             'rendering/RenderMarquee.h',
             'rendering/RenderMedia.cpp',
             'rendering/RenderMedia.h',
@@ skipping 2353 matching lines @@
                 'bison_exe': 'bison',
                 # We specify a preprocess so it happens locally and won't get
                 # distributed to goma.
                 # FIXME: /usr/bin/gcc won't exist on OSX forever. We want to
                 # use /usr/bin/clang once we require Xcode 4.x.
                 'preprocessor': '--preprocessor "/usr/bin/gcc -E -P -x c++"'
               }],
          ],
     }
 }