Add prerender throttle for about:srcdoc.

The associated issue occurs, in part, because cross-process srcdoc
iframes are not accounted for at present by the prerender throttle.
They may cause proxies to be unexpectedly be created, leading to the
CHECK in ~BrowsingContextState failing.

This CL updates the throttle to deal with cross-process srcdoc frames,
and adds a test that makes sure they still render correctly.

Bug: 1470312
Change-Id: I3c43d9552ea4a9662c7072b5c99e16c4e50be9b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4904773
Commit-Queue: James Maclean <wjmaclean@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210921}