libchrome: Use different condition to build libmojo. New USE flag "mojo" is being introduced, so libmojo build can depend on it. The dependency between cyrpto and mojo will be handled in ebuild. BUG=chromium:910002 TEST=Build locally. Change-Id: Ib127d22d6a94491efaca1a64953727ef7a1d0b40 Reviewed-on: https://chromium-review.googlesource.com/1454004 Commit-Ready: Hidehiko Abe <[email protected]> Tested-by: Hidehiko Abe <[email protected]> Reviewed-by: Ryo Hashimoto <[email protected]>