TimelinePanel: introduce StackView and use it instead of _timelineMemorySplitter

Source/devtools/devtools.gypi

 #
 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #         * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #         * Redistributions in binary form must reproduce the above
@@ skipping 287 matching lines @@
         'devtools_timeline_js_files': [
             'front_end/MemoryStatistics.js',
             'front_end/PieChart.js',
             'front_end/TimelineFrameController.js',
             'front_end/TimelineModel.js',
             'front_end/TimelinePresentationModel.js',
             'front_end/TimelineOverviewPane.js',
             'front_end/TimelineEventOverview.js',
             'front_end/TimelineFrameOverview.js',
             'front_end/TimelineMemoryOverview.js',
+            'front_end/StackView.js',
             'front_end/TimelineView.js',
             'front_end/TimelinePanel.js',
         ],
         'devtools_profiles_js_files': [
             'front_end/AllocationProfile.js',
             'front_end/BottomUpProfileDataGridTree.js',
             'front_end/CPUProfileView.js',
             'front_end/HeapSnapshot.js',
             'front_end/HeapSnapshotDataGrids.js',
             'front_end/HeapSnapshotGridNodes.js',
@@ skipping 143 matching lines @@
             'front_end/Layers3DView.js',
             'front_end/LayerDetailsView.js',
             'front_end/PaintProfilerView.js',
         ],
         'devtools_extension_api_files': [
             'front_end/ExtensionAPI.js',
             'front_end/DevToolsExtensionAPI.js'
         ],
     },
 }