Update the filled trace buffer % in title

During trace recording, stop overlay updates the % of trace buffer
filled. It is a warning to user to complete the usecase quickly as
once buffer is 100%, no more trace events will be recorded. But to
know the % of buffer filled, user has to switch back to tracing tab.

Instead of switching tabs back and forth we can show the buffer % in
title itself.

BUG=None

R=dsinclair