As part of painting directly from the LayoutNG fragment tree we need to break dependencies on InlineBoxes and LayoutItems (line layout API wrapper classes) as there won't be any InlineBoxes or LayoutObjects for inlines.

I can see three ways of doing this:
1) Change each call site to either access the LayoutObject/LayoutItem or the NGFragment.
2) Have NGFragments implement some of the LayoutItem API.
3) Introduce a new, more limited API, as a set of wrapper classes with separate implementations for legacy layout and LayoutNG.

The appropriate strategy will probably differ case by case but I suspect option 3 will be the easiest in the majority of cases.

The following files, outside of layout and paint, currently use the LayoutItem API:
Source/core/css/CSSCalculationValueTest.cpp
Source/core/css/CSSGradientValue.cpp
Source/core/css/CSSToLengthConversionData.cpp
Source/core/css/MediaValuesCached.cpp
Source/core/css/MediaValues.cpp
Source/core/css/MediaValuesDynamic.cpp
Source/core/css/MediaValuesInitialViewport.cpp
Source/core/css/resolver/StyleResolverState.cpp
Source/core/css/resolver/ViewportStyleResolver.cpp
Source/core/css/StyleEngine.cpp
Source/core/dom/Document.cpp
Source/core/dom/Element.cpp
Source/core/dom/FirstLetterPseudoElement.cpp
Source/core/dom/Text.cpp
Source/core/dom/TreeScope.cpp
Source/core/editing/CaretDisplayItemClient.cpp
Source/core/editing/commands/ReplaceSelectionCommandTest.cpp
Source/core/editing/DragCaret.cpp
Source/core/editing/FrameCaret.cpp
Source/core/editing/FrameSelection.cpp
Source/core/editing/RenderedPosition.cpp
Source/core/editing/SelectionController.cpp
Source/core/editing/SelectionModifierCharacter.cpp
Source/core/editing/VisibleUnits.cpp
Source/core/editing/VisibleUnitsLine.cpp
Source/core/editing/VisibleUnitsParagraph.cpp
Source/core/events/MouseEvent.cpp
Source/core/events/WebInputEventConversion.cpp
Source/core/events/WebInputEventConversionTest.cpp
Source/core/exported/WebDevToolsAgentImpl.cpp
Source/core/exported/WebDocument.cpp
Source/core/exported/WebFrameContentDumper.cpp
Source/core/exported/WebFrameTest.cpp
Source/core/exported/WebPagePopupImpl.cpp
Source/core/exported/WebPluginContainerImpl.cpp
Source/core/exported/WebViewImpl.cpp
Source/core/exported/WebViewTest.cpp
Source/core/frame/DocumentLoadingRenderingTest.cpp
Source/core/frame/Frame.cpp
Source/core/frame/FrameViewAutoSizeInfo.cpp
Source/core/frame/LocalFrame.cpp
Source/core/frame/LocalFrameView.cpp
Source/core/frame/RemoteFrameClientImpl.cpp
Source/core/frame/RemoteFrame.cpp
Source/core/frame/RemoteFrameView.cpp
Source/core/frame/VisualViewportTest.cpp
Source/core/frame/WebFrameWidgetImpl.cpp
Source/core/frame/WebLocalFrameImpl.cpp
Source/core/fullscreen/Fullscreen.cpp
Source/core/geometry/DOMMatrixReadOnly.cpp
Source/core/html/forms/TextControlInnerElements.cpp
Source/core/html/HTMLCanvasElement.cpp
Source/core/html/HTMLEmbedElement.cpp
Source/core/html/HTMLFrameOwnerElement.cpp
Source/core/html/HTMLImageElement.cpp
Source/core/html/HTMLObjectElement.cpp
Source/core/html/HTMLPlugInElement.cpp
Source/core/html/HTMLProgressElement.cpp
Source/core/html/ListItemOrdinal.cpp
Source/core/html/media/HTMLMediaElement.cpp
Source/core/input/EventHandler.cpp
Source/core/input/EventHandlingUtil.cpp
Source/core/input/MouseEventManager.cpp
Source/core/input/MouseWheelEventManager.cpp
Source/core/input/ScrollManager.cpp
Source/core/input/TouchActionTest.cpp
Source/core/inspector/InspectorCSSAgent.cpp
Source/core/inspector/InspectorDOMAgent.cpp
Source/core/inspector/InspectorLayerTreeAgent.cpp
Source/core/inspector/InspectorOverlayAgent.cpp
Source/core/page/DragController.cpp
Source/core/page/PrintContext.cpp
Source/core/page/scrolling/RootScrollerController.cpp
Source/core/page/scrolling/RootScrollerTest.cpp
Source/core/page/scrolling/ScrollingCoordinator.cpp
Source/core/page/scrolling/ScrollingCoordinatorTest.cpp
Source/core/page/TouchAdjustment.cpp
Source/core/scheduler/FrameThrottlingTest.cpp
Source/core/svg/SVGLengthContext.cpp
Source/core/svg/SVGTextContentElement.cpp
Source/core/testing/Internals.cpp
Source/core/testing/sim/SimCompositor.cpp
Source/modules/accessibility/AXInlineTextBox.cpp
Source/modules/accessibility/AXLayoutObject.cpp
Source/modules/accessibility/AXObjectCacheImpl.cpp
Source/modules/exported/WebAXObject.cpp
Source/modules/media_controls/elements/MediaControlElementsHelper.cpp

...and these use RootInlineBox directly:
Source/core/editing/LayoutSelection.cpp
Source/core/editing/RenderedPosition.h
Source/core/editing/SelectionModifierWord.cpp
Source/core/editing/VisibleUnits.h
Source/core/editing/VisibleUnitsLine.cpp
Source/core/editing/VisibleUnitsTest.cpp
Source/core/html/forms/TextControlElement.cpp