Chrome started requiring this dep to 'gn gen' on Android so
ANGLE needs it too.

Bug: angleproject:5839
Change-Id: Ia8df8520975ed426667f2c59f0fd7761652b92ab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2818141
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>