TimelinePanel: introduce StackView and use it instead of _timelineMemorySplitter

move recordsView construction code into a private method

Drive by fix: remove _popoverHelper and related code. We don't use it any more.

BUG=
R=aandrey@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/137853005

git-svn-id: svn://svn.chromium.org/blink/trunk@165121 bbb929c8-8fbe-4397-9dbb-9b2b20218538