GN: Make sure print function flushes stdout to avoid output delays
caused by buffered pipes.

R=dpranke@chromium.org

Change-Id: I02d459726488e8aeb126d34bcddd73b885833a98
Reviewed-on: https://chromium-review.googlesource.com/941207
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yngve Pettersen <yngve@vivaldi.com>
Cr-Original-Commit-Position: refs/heads/master@{#540011}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bbc17a39656082b3559be1ec6bdbf694b614ceb4