[DevTools] Network: initiator is 'other' after a JS navigation.

willSendRequest is async for navigation. With this patch we save true
initiator when navigation is scheduled.

BUG=272725

Review URL: https://chromiumcodereview.appspot.com/23092004

git-svn-id: svn://svn.chromium.org/blink/trunk@156227 bbb929c8-8fbe-4397-9dbb-9b2b20218538