Cache top-frame site value in Document to reduce visited links latency

When a blink::Document is constructed, this CL caches the value for top-
frame origin and stores it as a net::SchemefulSite. This has performance
benefits for the new partitioned :visited links model, as each link is
stored as a triple key: <link url, top-level schemeful site, frame
origin>. As a result, for each HTMLAnchorElement found in a Document, we
must perform the relatively expensive conversion from TopFrameOrigin()
to a net::SchemefulSite. By calculating this value once during
construction, we reduce the amount of time spent constructing the triple
key for each link in this Document.

I chose to use a PassKey to limit access to this cached value to
HTMLAnchorElementBase. When working in HTMLAnchorElementBase to
determine the triple-keys for each link, we have certain benefits, such
as a guarantee to be in a frame context where obtaining top-level site
produces a valid, non-null value, and a guarantee that the cached top-
level site will not become stale (if this happens the process is killed
before we could style a link incorrectly). Limiting access to this
function via passkey allows us to safely rely on these benefits.

Finally, from pprof analysis, we can see that this decreases the amount
of time spent in VisitedLinkState::DetermineLinkStateSlowCase by about
0.25%. For more information on why we are working to reduce latency in
partitioned :visited links, visit the bug below.

Bug: 372093900
Change-Id: Ifed3f1c0cb777efb765757b265213f7414962ca7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5949903
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: Kyra Seevers <kyraseevers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1372040}