Fenced frames: Disable "Save Link As..." after frame tree has network
cutoff.

Fenced frame can disable untrusted network access by calling
`window.fence.disableUntrustedNetwork()`. Once called, the fenced frame
and all its descendant iframes are not allowed make any network
requests. This is done by marking the fenced frame nonce as revoked for
network access. Requests that have such nonce will be cancelled.

In this CL, we disable "Save Link As..." in the context menu after a
fenced frame disables its network. Fenced frames are allowed to access
cross-site information, for example, shared storage get, after its
network is cutoff. It is possible to alter a link via javascript.
For example, the cross-site information about a user can be appended
to a link by using the "contextmenu" event. This way "Save Link As...",
and other download methods, can be used as a channel to leak cross-site
information. We must disable such channels.

In the next few CLs, we plan to do the same for:
"Save Image As..."
"Save Audio As..."
"Save Video As..."
"Save Video Frame As..."

Also, the other change we will make in the next few CLs is to append
the IsolationInfo, which has the nonce, to the download requests from
within the fenced frames. Requests that contain the nonce will be
cancelled by `URLLoader::CancelRequestIfNonceMatchesAndUrlNotExempted`.
This is needed to cancel download requests which are initiated just
before a nonce's network is revoked.

Bug: 1515599
Change-Id: I78f82b0d2b704beef69391ac42fd019f7cce5e0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5564165
Commit-Queue: Xiaochen Zhou <xiaochenzh@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Andrew Verge <averge@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1314703}