blinkpy: Update "layout test" in various places to "web test"

Bug: 843412
Change-Id: I1bd7de018b74d32c17ed36c3cf4b0e56a4b7b467
Reviewed-on: https://chromium-review.googlesource.com/c/1371404
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615751}
26 files changed