Profiler: Improve UI/UX for exiting "fiber durations" bar chart

[bvaughn]

Feedback from PR #1112

it took me a while to find how to get out of the "focused mode" because these buttons are pretty subtle. maybe the "close" button could be more visible when it's active?

[bvaughn]

React DevTools has been rewritten and recently launched a new version 4 UI. The source code for this rewrite was done in a separate repository and now lives in the main React repo (github.com/facebook/react).

Because version 4 was a total rewrite, and all issues in this repository are related to the old version 3 of the extension, I am closing all issues in this repository. If you can still reproduce this issue, or believe this feature request is still relevant, please open a new issue in the React repo: https://github.com/facebook/react/issues/new?labels=Component:%20Developer%20Tools