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 150 matching lines @@
             'front_end/SidebarTreeElement.js',
             'front_end/SimpleWorkspaceProvider.js',
             'front_end/SnippetStorage.js',
             'front_end/SoftContextMenu.js',
             'front_end/SourceFrame.js',
             'front_end/SourceMap.js',
             'front_end/SourceMapping.js',
             'front_end/SourcesPanelDescriptor.js',
             'front_end/Spectrum.js',
             'front_end/SplitView.js',
+            'front_end/StackView.js',
             'front_end/StatusBarButton.js',
             'front_end/StylesSourceMapping.js',
             'front_end/SuggestBox.js',
             'front_end/TabbedPane.js',
             'front_end/TempFile.js',
             'front_end/TestController.js',
             'front_end/TextEditor.js',
             'front_end/TextRange.js',
             'front_end/TextPrompt.js',
             'front_end/TextUtils.js',
@@ skipping 280 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'
         ],
     },
 }