[Aw WebExposed] Migrate to an instrumentation test

This moves (or copies) WebExposed and related dependencies from
WebViewLayoutTest to a new instrumentation test suite, WebExposedTest.

This change by itself is mostly just moving code without significantly
changing core implementation or behavior, which will instead be done in
subsequent changes.

The only significant change is the use of an updated rebaseline mode
extra, along with test_runner.py support, for running tests in
rebaseline mode. (A script for rebaselining into the checkout, similar
to run_tests_rebaseline.sh, is not added in this change.)

NO_IFTTT=Introducing cyclic LINT directives.

Bug: 381090604
Change-Id: I022215d58a57e92d6229678fae5a9d94f19f9058
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6054303
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Ashley Newson <ashleynewson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1395886}