Remove GN group backward-compat code.

This backwards-compat code automatically promoted a group's deps to be public if there were no public deps defined. This was done to make the transition to adding public/private deps easier.

Dependent on:
https://codereview.chromium.org/1356493004/ (chrome)
https://codereview.chromium.org/1351203002/ (blink)
https://codereview.chromium.org/1356723002/ (v8)

Review URL: https://codereview.chromium.org/1353023004

Cr-Original-Commit-Position: refs/heads/master@{#352071}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2616020c823cbf4cba3f1ad05f042952b8bacb05