Enabling 'gn check' on //webrtc/tools.

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2799633002
Cr-Original-Commit-Position: refs/heads/master@{#17542}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 129fc9ce9402c445184edcc1053b1864d9428771