GitLab

Trying to use ar to put a static lib inside a complete static lib doesn't
work. GN should enforce and document this restriction.

BUG=None
TEST=gn_unittests, including new test for this case.
R=brettw

Review URL: https://codereview.chromium.org/582863002

Cr-Commit-Position: refs/heads/master@{#295609}