TimelinePanel: introduce StackView and use it instead of _timelineMemorySplitter

move recordsView construction code into a private method

BUG=

Review URL: https://codereview.chromium.org/139463002

git-svn-id: svn://svn.chromium.org/blink/trunk@165152 bbb929c8-8fbe-4397-9dbb-9b2b20218538