Test that buffer usage reports are sent to the front-end when Timeline is recording trace events

This tests fix from https://codereview.chromium.org/430813006/

BUG=398787

Review URL: https://codereview.chromium.org/432423002

git-svn-id: svn://svn.chromium.org/blink/trunk@179496 bbb929c8-8fbe-4397-9dbb-9b2b20218538