Move use of DisplayItemList's vector behind an explicit DisplayItems interface.

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 626 matching lines @@
       'graphics/paint/ClipDisplayItem.cpp',
       'graphics/paint/ClipDisplayItem.h',
       'graphics/paint/ClipPathDisplayItem.cpp',
       'graphics/paint/ClipPathDisplayItem.h',
       'graphics/paint/ClipPathRecorder.cpp',
       'graphics/paint/ClipPathRecorder.h',
       'graphics/paint/ClipRecorder.cpp',
       'graphics/paint/ClipRecorder.h',
       'graphics/paint/DisplayItem.cpp',
       'graphics/paint/DisplayItem.h',
+      'graphics/paint/DisplayItems.cpp',
+      'graphics/paint/DisplayItems.h',
       'graphics/paint/DisplayItemCacheSkipper.h',
       'graphics/paint/DisplayItemClient.h',
       'graphics/paint/DisplayItemList.cpp',
       'graphics/paint/DisplayItemList.h',
       'graphics/paint/DisplayItemListContextRecorder.h',
       'graphics/paint/DisplayItemTransformTree.cpp',
       'graphics/paint/DisplayItemTransformTree.h',
       'graphics/paint/DisplayItemTransformTreeBuilder.cpp',
       'graphics/paint/DisplayItemTransformTreeBuilder.h',
       'graphics/paint/DrawingDisplayItem.cpp',
@@ skipping 370 matching lines @@
       ['os_posix==1 and OS!="mac"',
         {
           'platform_test_files': [
             'text/LocaleICUTest.cpp',
           ],
         }
       ],
     ],
   },
 }