Add browser tests for policy container

This adds a few browser tests for policy container [1]. With the
following objectives:
 - Verify that the policy container is properly populated (at this stage
   only the referrer policy is available within the policy container),
   from the header and from the meta tags.
 - Verify that the policy container is properly inherited in popups and
   iframes.
 - Verify that the inherited policy is the one applied.
 - Verify the behavior of policy container when an iframe is "created"
   by a sibling: Even if the script was executed by a sibling, the
   parent frame is always the creator.

[1] https://docs.google.com/document/d/16-bbA-DsO1l3vZnWYemRhcaUJZ8g8SZhMo99MmGFRSs/edit#heading=h.o51zkgcv0thc

Bug: 1130587
Change-Id: I37d4cbb9882f72ac041559f15fb06675bb88dd3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2457186
Commit-Queue: Pâris Meuleman <pmeuleman@chromium.org>
Reviewed-by: Antonio Sartori <antoniosartori@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Auto-Submit: Pâris Meuleman <pmeuleman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824377}

content/test/BUILD.gn

@@ -1110,6 +1110,7 @@ test("content_browsertests") {
     "../browser/renderer_host/navigation_request_browsertest.cc",
     "../browser/renderer_host/page_lifecycle_state_manager_browsertest.cc",
     "../browser/renderer_host/panel_rotation_browsertest.cc",
+    "../browser/renderer_host/policy_container_browsertest.cc",
     "../browser/renderer_host/render_document_host_browsertest.cc",
     "../browser/renderer_host/render_document_host_user_data_browsertest.cc",
     "../browser/renderer_host/render_frame_host_impl_browsertest.cc",