DevTools: NetworkPanel: don't show cryptic headers for inflight requests

Provisional headers bring internal header which disappears after finish:
'X-DevTools-Emulate-Network-Conditions-Client-Id'

BUG=

Review URL: https://codereview.chromium.org/528203002

git-svn-id: svn://svn.chromium.org/blink/trunk@181292 bbb929c8-8fbe-4397-9dbb-9b2b20218538