DevTools: NetworkPanel: add filter for running requests.

"Is:running" do thw magic.

BUG=419695

Review URL: https://codereview.chromium.org/621303002

git-svn-id: svn://svn.chromium.org/blink/trunk@183197 bbb929c8-8fbe-4397-9dbb-9b2b20218538