Update special handling for "LayoutTests/" paths in tests

- In web_tests, we mapped the keyword "file:///tmp/LayoutTests/" to
  the actual web_tests directory.  This CL updates the keyword to
  "file:///tmp/web_tests".

- In image decoder tests, we mapped the keyword "LayoutTests/" to
  the actual web_tests directory.  This CL updates the keyword to
  "web_tests/".

Bug: 843412
Change-Id: I6f4ef498a9052a701aa2211a180d87781f45d8dc
Reviewed-on: https://chromium-review.googlesource.com/c/1351195
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611055}
24 files changed