blinkpy: Rename function names and variable names related to "layout test directory" to "web_test"

Also, update symbols and comments around them.
This CL has no behavior changes.

Bug: 843412
Change-Id: Ic1c2381e5d431fdedf5978d510cc9018e101d70d
Reviewed-on: https://chromium-review.googlesource.com/c/1370157
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615415}
49 files changed