Add TestActivationManager test helper

TestActivationManager is a new test helper for controlling page
activating navigations like prerender and BFCache.

Currently, tests use TestNavigationManager to control activating
navigations; however, this can be difficult and confusing since it uses
NavigationThrottles to pause a navigation, which aren't used in a page
activating navigation.

https://crrev.com/377fb3e653c9493d2 tried to make this work by adding a
CommitDeferringCondition in TestNavigationManager; however, this broke
for prerendering after https://crrev.com/6c5f39bd1022 moved prerender
activations to run CommitDeferringConditions before navigation start.
Prerender tests then used MockCommitDeferringConditions to do this
manually.

This CL adds TestActivationManager which uses CommitDeferringConditions
installed via a global testing hook in CommitDeferringConditionRunner.
It provides a less error-prone interface by only handling activations;
CommitDeferringConditions can run either before RequestStart or after
WillProcessResponse which makes the TestNavigationManager WaitFor___
interfaces awkward to use.

To show how TestActivationManager works, this CL converts a few tests
that were using TestNavigationManager or more manual methods to stop an
activating navigation. In a followup, we'll add a DCHECK in
TestNavigationManager that the navigation isn't page activating so that
all tests use the correct manager for a navigation.

Also do some minor cleanups on MockCommitDeferringCondition to make it
more ergonomic and harder to use incorrectly.

Bug: 1199724
Change-Id: I6cbc55601e7ea3a5524fbe9701f26e9d757944d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3450737
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#972589}