GitLab

This picks up:

    Roll the GN binaries to #318791.

    Roll the GN binaries from 317120:318791 (from git 74add9fc
    to 3d7a60db).

    This fixes 'gn clean' and picks up some new features for
    introspections (see #317159).

TBR=brettw@chromium.org
BUG=430215

Review URL: https://codereview.chromium.org/965963002

Cr-Commit-Position: refs/heads/master@{#318952}