NavigationThrottleRunner2: content::NavigationThrottleRegistry

This CL introduces a NavigationThrottleRegistry.
CreateThrottlesForNavigation() is changed to use this interface to
register a NavigationThrottle for a new navigation.

We still handle the returned throttles vector to keep the legacy
interface available. But, once all the existing NavigationThrottle are
migrated to use the NavigationThrottleRegistry, we will remove the
legacy way, and the signature will be changed to return void.

With this interface, we can implement several optimization step by step
without changing the existing API signature further more. For instance,
we can provide following features via the new registry interface;
- fast access paths to check navigation attributes
- per-event registration to optimize event loops

Eventually, I will introduce a new NavigationThrottleRunner class. As we
want to share some common implementation among the original runner and
the new runner during the experiment, I will move the common logic to
this registry class and both runners will own it to share the
implementation.

Essential changes are in the NavigationThrottleRunner, and
//content/public. Others are call side mechanical changes.

Bug: 412524375
Change-Id: Ifda11e0f5cf446135c9ab34df025573346c7639c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6478853
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Auto-Submit: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454542}