GitLab

- Use absolute path to make filters work.
- Fix description to account for fact that we just want developer
  to add GN configuration and we build the target for them.
- Fix missing target in pdfium command line.
- llvm directory can be cleared when target_os change, but
  cr_coverage_revision stays intact. So, check for coverage
  tool binaries, otherwise we will crash on file not found.

R=mmoroz@chromium.org,liaoyuke@chromium.org

Bug: 
Change-Id: I0ce23c09863323af2d03561a232cb9e614493506
Reviewed-on: https://chromium-review.googlesource.com/814495
Commit-Queue: Abhishek Arya <inferno@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522464}