[test_runner] Use DidStartLoading for tracking loading frame

... instead of DidStartProvisionalLoad. This is a preparation
for removing DidStartProvisionalLoad in some (later in all) cases.

This change makes test_runner::WorkQueue interact differently with
anchor navigations, because WorkQueue is posting a task, and the
timing is different. Marking these anchor navigations as "loading"
scripts fixes the issue, and seems natural.

Bug: 855189
Change-Id: Ie46a7886926cbbda4c2cd7f8697c22c5f76ef1c7
Reviewed-on: https://chromium-review.googlesource.com/c/1267348
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597997}
5 files changed