Revert "TimelinePanel: introduce StackView and use it instead of _timelineMemorySplitter"

This reverts commit b5c27b516609da2e9fa13187f6eb2a31e50f529c.

BUG=
TBR=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/130483006

git-svn-id: svn://svn.chromium.org/blink/trunk@165128 bbb929c8-8fbe-4397-9dbb-9b2b20218538