[mparch] Allow subframe navigations during activation in the Prerender

Given that the FrameTreeNode does not change after activation for
sub-frame navigations it *should* be safe to allow them. One possible
caveat is that NavigationRequest might not support the change in
associated NavigationController (owned by FrameTree), in particular not
handing navigation entries correctly.

We will need to have a closer look at this but there is a bug already
for that. In the meantime we can unblock the removal of multiple
WebContents implementation.

Note that the affected tests (DeferCrossOrigin*),  are currently
disabled due to flakiness unrelated to what this patch fixes.

Bug: 1154501
Bug: 1190644

Change-Id: Idb3c5f824513c88f92711b7b92befaf98afb43a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2772996


Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#866186}