DevTools: [Timeline] Reset Memory Overview pane when clear button is clicked

Contributed by pan.deng@intel.com

BUG=263683

Review URL: https://chromiumcodereview.appspot.com/20066005

git-svn-id: svn://svn.chromium.org/blink/trunk@154975 bbb929c8-8fbe-4397-9dbb-9b2b20218538