Remove 'LayoutTests' in content/ and third_party/blink as much as possible

Remove 'LayoutTests' in testing code, comments, documents, and OWNERS
in files other than third_party/blink/web_tests.

This CL doesn't change symbol names in source code.
This CL doesn't have any behavior changes

Bug: 843412
Change-Id: I6eeda7b12ba3e484dff7559d21a9c13a3214795f
Reviewed-on: https://chromium-review.googlesource.com/c/1350372
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610785}
77 files changed