Remove android build dir nesting restriction

It appears that android builds no longer have to be nested exactly two
levels under //. A build with one level works, so the assert in gn is
no longer helpful.

BUG=412935
R=agrieve@chromium.org

Review-Url: https://codereview.chromium.org/2544493002
Cr-Commit-Position: refs/heads/master@{#435410}
1 file changed