headless: Require the user to pass in an initial URL

Require the user to pass in an initial URL when creating a
HeadlessWebContents. This is to ensure that we have a navigation that
initializes the renderer, making it possible to inspect the tab.

BUG=595353,546953

Review URL: https://codereview.chromium.org/1858403003

Cr-Commit-Position: refs/heads/master@{#385476}
11 files changed