[DevTools] Network: remove confusing "Pending" label for request type.

When mime type is not known (for example, when request was cancelled)
we show "Pending" label. That confuses users.

BUG=301559

Review URL: https://codereview.chromium.org/25631004

git-svn-id: svn://svn.chromium.org/blink/trunk@158813 bbb929c8-8fbe-4397-9dbb-9b2b20218538