Extract tree root responsibility out of HTMLImportsController.

Source/core/core.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',
             'animation/AnimationNode.idl',
             'animation/AnimationTimeline.idl',
             'animation/Timing.idl',
@@ skipping 2663 matching lines @@
             'html/imports/HTMLImport.cpp',
             'html/imports/HTMLImport.h',
             'html/imports/HTMLImportChild.cpp',
             'html/imports/HTMLImportChild.h',
             'html/imports/HTMLImportChildClient.h',
             'html/imports/HTMLImportLoader.cpp',
             'html/imports/HTMLImportLoader.h',
             'html/imports/HTMLImportState.h',
             'html/imports/HTMLImportStateResolver.cpp',
             'html/imports/HTMLImportStateResolver.h',
+            'html/imports/HTMLImportTreeRoot.cpp',
+            'html/imports/HTMLImportTreeRoot.h',
             'html/imports/HTMLImportsController.cpp',
             'html/imports/HTMLImportsController.h',
             'html/imports/LinkImport.cpp',
             'html/imports/LinkImport.h',
             'html/parser/AtomicHTMLToken.h',
             'html/parser/BackgroundHTMLInputStream.cpp',
             'html/parser/BackgroundHTMLInputStream.h',
             'html/parser/BackgroundHTMLParser.cpp',
             'html/parser/BackgroundHTMLParser.h',
             'html/parser/CSSPreloadScanner.cpp',
@@ skipping 700 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',
         ],
     }
 }