Shared Storage: Add browser tests for iframe attribute

We add browser for our implementation of writing to
shared storage from the new `HTMLIFrameElement` IDL/content attribute
`sharedStorageWritable`/`sharedstoragewritable`.

This is part of a project to add support to writing to shared storage
from response headers. See
WICG/shared-storage#70 for more information
and https://crrev.com/c/4691573 for a prototype for adding image
and iframe attributes.

Bug: 1434529,1218540
Change-Id: I3cfef638a284e2e3d36e4142c87c0423b29235f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4718688
Reviewed-by: Yao Xiao <yaoxia@chromium.org>
Commit-Queue: Cammie Smith Barnes <cammie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1187554}