Show allocation stack trace for selected heap object

Retainers view is replaced with a tabbed pane. One tab is the old retainers view, the second one displays allocation stack trace.

BUG=277984
R=alph@chromium.org

Review URL: https://codereview.chromium.org/220123015

git-svn-id: svn://svn.chromium.org/blink/trunk@170760 bbb929c8-8fbe-4397-9dbb-9b2b20218538