GN: Fix for chromeos building.

Tested with the following command lines:

$ gn gen --args='os="chromeos"' out_gn_chromeos/Debug

and

$ gn gen --args='os="android" cpu_arch="arm"' out_gn_android/Debug

See gn-dev mailing-list thread discussion:

https://groups.google.com/a/chromium.org/d/topic/gn-dev/Mq2ycNv4vRg/discussion

BUG=None
TEST=see above
R=brettw@chromium.org,eae@chromium.org

Review URL: https://codereview.chromium.org/846073005

Cr-Commit-Position: refs/heads/master@{#311193}
1 file changed