scanning: Use test browser proxy getArgs() in tests

Using methodCalled() to store the values passed to the browser proxy
then calling getArgs() to retrieve them is the correct way to verify the
values. It lets us remove hidden asserts out of the browser proxy.

Also remove methodCalled() from functions where it's not needed.

Change-Id: I6e2f7bba5347c01ec3f06f3d8f4f958f4fde15ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2873002
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880046}