DevTools: NetworkPanel: remove NetworkBaseCalculator.

This class has single descendant, so it is useless.

Also added JSDocs, and removed unused code.

BUG=

Review URL: https://codereview.chromium.org/484953004

git-svn-id: svn://svn.chromium.org/blink/trunk@180783 bbb929c8-8fbe-4397-9dbb-9b2b20218538