[ios] Re-enable flaky navigation restore egtests.

Both testRestoreHistory tests were emulating a background/kill/restore
by simply disabling/enabling web usage.  Doing a tabgrid close all /
undo is more similar.

Upon EG2 migration, this method can be removed, and the test can do
a proper background/kill/restore.

Bug: 1011791
Change-Id: I1f3074917747551833ef090e2da598674476de77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846072
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703475}