TimelinePanel: remove unnecessary container from MemoryStatistics view.

MS view was View and had only one child element SidebarView.

BUG=326121

Review URL: https://codereview.chromium.org/145523003

git-svn-id: svn://svn.chromium.org/blink/trunk@165706 bbb929c8-8fbe-4397-9dbb-9b2b20218538