Add an assert that TracedValue builder closes only most recently opened container

When an array or a dictionary is ended we check that there are no other open containers on top of it.

BUG=None

Review URL: https://codereview.chromium.org/356013002

git-svn-id: svn://svn.chromium.org/blink/trunk@177095 bbb929c8-8fbe-4397-9dbb-9b2b20218538