DevTools: Split NetworkPanel.js

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=185306

Review URL: https://codereview.chromium.org/722683003

git-svn-id: svn://svn.chromium.org/blink/trunk@185384 bbb929c8-8fbe-4397-9dbb-9b2b20218538