GitLab

1. Remove the links of deleted files
  - docs/common_build_tasks.md
  - docs/shift_based_development.md
  deleted in https://chromium-review.googlesource.com/679054
  - docs/subtle_threading_bugs.md
  deleted in https://chromium-review.googlesource.com/897782

2. Remove Old Chrome OS build instructions and
   add Chrome OS build instructions in Checking Out and Building
  - docs/old_chromeos_build_instructions.md
  renamed in https://chromium-review.googlesource.com/861043

3. Remove the link of an unidentified file
  - docs/testing/test_browser_ui.md
  can't find the history of the file.

Bug: N/A
Change-Id: I4c6d3e6fbe442ad79905c94b94627ad8baf251dd
Reviewed-on: https://chromium-review.googlesource.com/1029396
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554686}