GitLab

* Add an implementation of NavigationPolicyThrottle to WebEngine
* Add NavigationPolicyHandler, which keeps track of throttles and checks
  if navigations need to be passed on to the client
* Add a FakeNavigationPolicyProvider for tests

Test: NavigationPolicyThrottleTest
Bug: 1117601
Change-Id: I0bdf94345b10d829bea4aeb85f74204970a97239
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519039
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826625}