Add a NavigationThrottle to the public content/ interface

Add a NavigationThrottle to the public content/ interface

This CL adds a NavigationThrottle class to the public content/ interface. A NavigationThrottle is used to control the flow of navigations. It lives entirely on the UI thread. Eventually, all components that want the functionality of a ResourceThrottle for navigations should transition to a NavigationThrottle: the new architecture for navigations (browser-side navigation) will not support ResourceThrottles for main resources load. See https://docs.google.com/document/d/1ICLLQoC9EsZ-bWH4ZKRhPCIoZKn6pOj02SlGl6SKH6Y/edit?pli=1#heading=h.fmxjmgvbgg7x for the design doc.

This Cl also transition the InterceptNavigationresourceThrottle to the Navigationthrottle model.
BUG=504347

Review URL: https://codereview.chromium.org/1269813002

Cr-Commit-Position: refs/heads/master@{#350092}