Move GraphicsLayerUpdater::rebuildTree into its own file

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 1662 matching lines @@
             'rendering/compositing/CompositingLayerAssigner.cpp',
             'rendering/compositing/CompositingLayerAssigner.h',
             'rendering/compositing/CompositingPropertyUpdater.cpp',
             'rendering/compositing/CompositingPropertyUpdater.h',
             'rendering/compositing/CompositingReasonFinder.cpp',
             'rendering/compositing/CompositingReasonFinder.h',
             'rendering/compositing/CompositingRequirementsUpdater.cpp',
             'rendering/compositing/CompositingRequirementsUpdater.h',
             'rendering/compositing/CompositingState.h',
             'rendering/compositing/CompositingTriggers.h',
+            'rendering/compositing/GraphicsLayerTreeBuilder.cpp',
+            'rendering/compositing/GraphicsLayerTreeBuilder.h',
             'rendering/compositing/GraphicsLayerUpdater.cpp',
             'rendering/compositing/GraphicsLayerUpdater.h',
             'rendering/compositing/RenderLayerCompositor.cpp',
             'rendering/compositing/RenderLayerCompositor.h',
             'rendering/line/BreakingContext.cpp',
             'rendering/line/LineBreaker.cpp',
             'rendering/line/LineBreaker.h',
             'rendering/line/LineWidth.cpp',
             'rendering/line/LineWidth.h',
             'rendering/line/TrailingObjects.cpp',
@@ skipping 1717 matching lines @@
             'loader/MixedContentCheckerTest.cpp',
             'page/NetworkStateNotifierTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }