Add IsOutermostMainFrameRequest() to IsolationInfo

This CL adds a new function to IsolationInfo that identifies when an
IsolationInfo is associated with a primary main frame request.

This differs from the existing IsMainFrameRequest() by considering if
the IsolationInfo has a nonce. Practically this means that
IsOutermostMainFrameRequest() only returns true for requests for a
"real" top-level frame. As opposed to MPArch frames, such as Fenced
Frames, which behave as if they're main frames but are embedded within
an outer main frame.

(Motivated by
https://chromium-review.googlesource.com/c/chromium/src/+/6190847/comment/e92ea999_aa21ea4e/)

Bug: 40725781
Change-Id: Ic5f60a635f7394c355fc98604ca4f6772d887b3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6226567
Commit-Queue: Steven Bingler <bingler@chromium.org>
Reviewed-by: mmenke <mmenke@chromium.org>
Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1415783}