DevTools: NetworkPanel: get rid of stripes gradient.

Rendering gradient consumes considerable amount of time.

BUG=316092

Review URL: https://codereview.chromium.org/440853002

git-svn-id: svn://svn.chromium.org/blink/trunk@180449 bbb929c8-8fbe-4397-9dbb-9b2b20218538