This is very similar to the existing rule for a GTAGS file, and reuses
parts of it (the gtags.files file).

R=ahaas@chromium.org, mstarzinger@chromium.org, titzer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1873253002

Cr-Commit-Position: refs/heads/master@{#35435}